June 19, 2010:
Revised: v2.0

ZL2PD Electronic Sand Timer

Originally designed in 1993 by ZL2PD, this design is republished on this site with the kind permission of ELEKTOR magazine (The original English version was published in Elektor across Europe in various languages back in June 1995)


This is a microprocessor based version of the original sand and glass type sand timer. I designed this way back in 1993. More versatile than the original 3 minute only version, this sand timer, although still based on silicon, can be set to run from 1 to 99 minutes. Great for children!

This article was originally published in ELEKTOR magazine during 1995, the English version appearing in June of that year, and in a variety of other European language versions of the magazine around the same period. It uses the Philips 87C751 microprocessor which has since been declared obsolete. However, there are bound to be a few in the odd junk box, so I asked ELEKTOR if I could republish this article on my website to permit those chips to be used in something useful. And so, here's that article, reproduced with their kind permission.

The 87C751 device was one of the smallest members of the 8051 family, and with very minor changes in the code, the software will run on any similar chips, from a standard 87C51, or one of the Atmel 89C1051, 89C2051 etc family. The circuit board will need amendment to use these other devices since they are not able to be plugged straight into the board.        

The Rationale for the Design       

Young children find it hard to read the time. The long hand and the short hand on the clock are frequently mixed up. Digital clocks can be even more confusing for children, with numbers being read back to front, even upside down! Curiously, the difficulty with reading the right time seems to rise to a peak around bedtime, or when something important has to be done.

An ordinary sand timer is a great solution. It clearly shows how much time has elapsed, and how much is left. But for avid readers of books, especially the author's children, the traditional 3 minute sand timer was not nearly long enough.

This electronic sand timer was the solution I came up with to solve the problem. It is programmable, allowing for timed durations of up to 99 minutes. It is also very colourful. Even when not in use, it serves as a useful 'night light' for children who need some help in getting off to sleep. In operation, it simulates the falling sand grains of the original, and allows children to quickly judge how long it has to run before the lights go out. Or, to be truthful, the time to run until the next appeal for more time to higher authorities.

To keep the project as simple as possible, the sand timer only uses one chip, a small 8051-family single chip microcontroller. The chip drives the display, reads the switches, sounds the alarm, and counts the time. With only a few parts required to this busy little chip, the sand timer is also quick and easy to make.        

The original prototype is still in use. Pictured at the top of this page, the prototype has a slightly different layout to that published by Elektor. It has pushbuttons on the right hand side, and ta slide switch for turning the power on and off. 

The 87C751 Microcontroller    

The microcontroller used in the sand timer is from the 8051 family. Originally developed by Intel, versions of the 8051 are also made by a number of other companies. One of these, Philips Semiconductors, produce a range of 8051 variants to meet different applications, and the sand timer uses one of the smallest family members, the 87C751. It's no longer available, but similar compatible devices are available from other suppliers.

As for almost all single chip microcontrollers, the 87C751 includes EPROM (erasable programmable read-only memory), RAM (random access memory), CPU (central processing unit) functions and I/O (input/output) pins. The micro has three ports with a total of 16 I/O lines, two interrupt pins, several special timer input pins, in addition to two versatile internal timers. The Philips "I2C" special serial control bus is also implemented on the chip, for use with the wide variety of specialised interface chips available in the I2C family.

Manufactured in CMOS technology, the 87C751 has a low current consumption and heat dissipation. The chip can be placed in several idle modes where it waits for some signal requiring it to act. In this quasi-sleep mode, it only draws a few micro amps of current.

The I/O pins can be programmed into a variety of modes. Some pins share functions, and may be used either for direct I/O or for some interrupt driven function. The I2C serial bus system pins on the device may be used to drive specialized chips for use as infra red remote control, analog I/O, additional parallel I/O, or even complete radio synthesizers.

The I2C interface is not used in this design. Instead, the chip directly drives an array of LEDs which simulate the falling sand in a traditional sand timer. Through fast multiplexing of the display, the current requirements of the timer are minimised. The 87C751 is a current miser in any case, consuming less than 20 mA for most of the time. Overall, the LED display that represents the sand timer adds another 5 mA to the current drain.

The Design   

The circuit diagram of the electronic sand timer is shown below. All of the instructions executed by the microcontroller are contained in its internal read only programmable memory. These instructions guide it step by step through all of its functions. They are accurately timed by the microprocessor's crystal oscillator. The crystal used in the sand timer has a frequency of 11.0592 MHz. This is a standard frequency for MCS 51 processors.

The microprocessor handles the display, the user controls, and the alarm. The display consists of 17 LEDs of various colours, driven by the micro through a 6x3 row and column matrix.

Each row is driven from one port, while the three columns are driven by three other port pins. All pins are buffered via individual transistors due to the LED drive current requirements. These exceed the individual pin drive capabilities of the 87C751 chip.

The display is used in three different ways. Its prime role, of course, is to emulate the sand in a sand timer. It also provides a simple colourful random display to attract users when not in use as a timer. The third use of the display is to help program the timeout period. In that mode, the LEDs are used to display the alarm time, from 1 to 99 minutes, displaying the incremented time when ever the SetTime button is held down.

The display consists of only 17 LEDs so to display the numerals from 1 to 99 takes a little imagination, particularly with the digit '4'. Despite this limitation, the time display is quite clear and easy to read. The LEDs display the alarm time when the SetTime button is pressed. The display then shows the programmed time briefly before reverting to the random display again.    

The micro contains two timers, one of which operates almost continuously in the background to produce a heartbeat-like periodic timer for the clock. This 'tick' clock is used to control the multiplexing of the display. When the timer is running, it is also used to measure the required alarm time. While the software allows timing down to a resolution of milliseconds, this application only requires timing to the nearest minute, and this is the resolution used within the timer itself.

The sand timer is operated by just two controls, the SetTime and StartStop buttons, which are connected directly to the microcontroller. The microprocessor periodically checks their status, debounces them, and waits for them to be released before continuing. The SetTime button allows the required timeout period to be set. The Start/Stop button, as the name implies, starts and stops the timer. Times from 1 to 99 minutes may be entered, and errors such as trying to set the timer for 0 minutes are automatically detected and ignored.

To speed up this process, since setting the timer via the SetTime key takes a minute or so while the timer display increments through the available times, the timer reads the user's preferred time from one of the ports when it is first started. The preferred (preset) time is set with the aid of an 8-way DIP switch, S3. For most applications, the combination of the preset time and the SetTime button will be more than adequate.

The timer
flashes all the LEDs and by emits a warble from the piezo speaker to show that the required time has elapsed. The use of the piezo speaker maximises the volume while minimising current consumption. This also avoids the alternative requirement for a driver transistor and regular speaker. Since this timer may well be used for other purposes, one other port pin has been set aside for driving external devices directly. This pin may be buffered and used to drive an external relay or other devices.

The timer is powered from a single 9 V battery. The battery voltage is stepped down a three pin regulator type 7805, which delivers the 5 V supply required by the microcontroller. Decoupling capacitors are used around the regulator to ensure stability and reduce any possible interference.


The design and layout of the printed circuit board is shown below. (Note: This is the Elektor PCB layout and differs slightly from the prototype pictured above)

Start by fitting the wire links on to the board. Check your work very carefully, because if you forget to fit one, the timer is not likely to work, even if you do fit the other parts in the correct way.

Next, add the capacitors, the resistors, the diodes and the DIP switch. Check the polarity of the electrolytic capacitors and the diodes.
Insert the socket for the micro. If you can not get hold of a narrow (0.3in. wide) 24 pin IC socket, use three 8pin sockets instead. Make sure the notch is at the side indicated on the component overlay. Then carefully solder the crystal. While using the 11.0592 MHz crystal suggested will produce the best timer accuracy, any crystal within 250 kHz from that frequency will be acceptable. For example, if you have an 11.0 MHz crystal available, that will be suitably accurate.

Install the transistors for the display, and then the LEDs. Since the display uses three NPN transistors (e.g. BC547B) and six PNP transistors (e.g. BC557B), there is room for confusion here. Check before soldering.

The LEDs must be mounted at a height of about 6 mm. The simplest way to do this is to get hold of some drinking straws, and cut off pieces with a length of 6 mm. Insert one of the LED legs through it, and then solder them into the PCB. Again, watch to make sure you install them the correct way around.

The two push buttons should be mounted a little higher than the LEDs. Plastic PCB spacers are suitable for that purpose.

Note: ELEKTOR's description (above) describes the method used to mount the LEDs and pushbuttons about 6mm off the PCB. This was done to allow for the height of the microprocessor and socket for this chip which would otherwise prevent the buttons from sitting correctly through the front panel. My prototype had the LEDs mounted down directly onto the PCB, as were the switches. My switches were high enough to allow the user to press them through holes in the front panel. If you use smaller switches, these may also need to be mounted slightly off the PCB.

Also note that the ELEKTOR overlay and parts list noted the transistors as T1-T9 while the circuit diagram shows these as Q1 - Q9.    

As shown in the above layout diagram, the sand timer will "wake up" and configure itself for a ten minute timeout, but using the DIP switch and diodes allows other "wake-up" or "alarm" times to be set. The DIP switch (S3) settings are shown in the following table. They allow times from 1 to 99 minutes to be set.        

Mount the 78L05 voltage regulator next. You can use either this small TO-220 sized regulator or the larger and more commonly available 7805 regulator. Add the battery wires, and once more check that all the components are fitted correctly, in the right place, and the right way around. Do not insert the microcontroller yet!

Connect a 9 V battery and measure the voltage between pin 24 (positive) and pin 12 (negative or common) of the microcontroller socket. The meter should display between 4.5 V and 5.5 V.

If the voltage is too high or too low, check and fix the fault. The fault is likely to be either a poorly soldered joint, or the regulator has been fitted the wrong way around. Do not proceed until your timer passes this test. Now disconnect the battery.

The next test checks the display to make sure that the LEDs are connected correctly, and the driver transistors are functioning. Temporarily connect pin 5 of the microcontroller socket to ground. Reconnect the battery. LEDs D9, D11 and D13 should light. Temporarily link, one at a time, pins 6, 7 and 8 to ground. This should turn each of the, LEDs off in turn.

Disconnect the ground link from pin 5, and connect it to pin 4 to test the next row. Then repeat this test again for subsequent rows, using pins 3, 2, 1 and 23. If any individual LED does not light, check to see that it has been inserted into the PCB correctly. If an entire column or row of LEDs does not function correctly, check the relevant buffer transistor. Note that one row has just two LEDs.

Now set the DIP switches in accordance with the preset timeout period you want. The switches in block S3 are programmed in BCD (binary coded decimal) to represent the tens and units of the desired time. If the switch is omitted, the timer will initialise itself with a default period of 10 minutes. Table 1 (above) indicates the switch settings.

For example, to program a timeout period of 15 minutes you require a binary pattern 0001 0101. This is achieved by leaving switches S3(1), S3(2), S3(3), S3(5) and S(7) open, and closing S3(4), S3(6) and S3(8). The pattern on the DIP switch then looks as follows:

Example setting for S3 to set a 'wake-up' value of 15 minutes for the timer    
To help you with the orientation, switch S3(1) is connected to diode D1, and S3(8) to diode D8.

It should be noted that the timer reads the switches only when the power is turned on. If you want to change the preset value, you will need to switch off the power first and then change the switch settings. Then turn the power back on again. Alternately, you can use the 'Set Time' button method described below.

Next, install the 87C751, observing precautions against static electricity discharges which may damage the device. Also make sure the 87C751 is fitted the right way around on the board. The micro has a notch at the top which should be at the same end as the notch printed on the PCB overlay and the notch on the socket.

Attach the battery. The display will remain blank for about half a second before bursting into life with the random light display. This will run as long as power is connected and the timer itself is not set running.

Press the SetTime button, and release it. The display will briefly change to show the preset time you encoded with the DIP switch. If you did not fit the switch, the timer will display the default time of 10 minutes. If you continue to hold the SetTime button, the timer will slowly increment the Alarm Time, displaying each number in turn, and starting at the preset value read from the DIP switch. After displaying 99 minutes, the timer will start again at 1 minute. A timer setting of '0' or more than 99 minutes is not possible.

Once the time has been set, the timer can be started. Press and release the Start/Stop button. The timer display will change again, this time to the simulation of the sand timer. The 'sand grains' will fall bit by bit, changing periodically. The display changes at a rate of 2.5 seconds per minute of alarm time. For example, if the alarm time is set at two minutes, the display will change every five seconds. For an alarm time of 15 minutes, the display will change every 37.5 seconds.

If the Start/Stop button is pressed during this mode, the timer display will flash and revert to random mode, halting and resetting the timer. Pressing the SetTime button has no effect in this mode. You must stop the timer to reset the alarm time.

At the end of the preset time, the alarm will be heard, and seen. This consists of repeated display flashing and rising beeps of sound from the piezo speaker. They will continue for about a minute, or until the Start/Stop button is pressed. This resets the timer back to the random mode.

All of this may sound a little complex, but in fact the operation of the timer is quite straightforward. A few minutes of experimentation will demonstrate all the functions and clarify any confusion.    

Making the Sand Timer's Plastic Stand

The timer can be mounted into a suitable enclosure, or on the little perspex stand shown in the photo. This is made from a single sheet of 2 mm or 3 mm thick smoked perspex which has been bent to form the stand. The best way to bend it is to use a hot air gun or electric paint stripper, the latter being the one I found easiest to use.

You will need to experiment to see how long to heat up your piece of perspex before it will bend. If you heat the perspex too much, small bubbles will appear in the plastic, and this ruins the effect you are aiming for. If the perspex has protective paper on it, remove it before trying to bend it.

Place the perspex on a suitable surface, and blow the hot air over the area of the fold from about 5 to 10 cm away. Play the hot air over the area for about 10 seconds, then carefully try to bend the perspex over a sharp comer. It is recommended to use several pieces of plywood; one as the sharp edge, the other to push against the hot perspex.

You will probably need several tries before you get the hang of this, so be prepared to experiment. Do not use any sort of flame! Perspex is flammable!

The sheet should be drilled as shown in Fig. 3 before bending it. Mounting of PCB on to the stand is simple with the aid of four plastic PCB spacers. Securing the battery and connecting it to the board completes the construction of the sand timer.

Finally, it is, of course, possible to use a mains adaptor with an output voltage between 9 V and 12 VDC instead of the 9V battery. The average consumption of the timer is about 25mA.

Parts List

  R1        47k
  R2, R4, R6, R8, R10, R12, R14, R15, R16     10k
  R3, R5, R7, R9, R11, R13      100k
  R17 - R33      680 ohms

  C1, C5, C6        10uF, 25V
  C2, C3               22pF ceramic
  C4                      100nF ceramic

  D1 - D8             1N4148 small signal silicon diode
  D9, D13, D17, D21, D25                     Green LED
  D10, D11, D12, D22, D23, D24          Red LED
  D14, D15, D16, D18, D19, D20          Yellow LED
  D26                   1N4001 1A silicon diode
  T1 - T6             BC557B
  T7, T8, T9        BC547B
  IC1                   Philips 87C751 microcontroller
  IC2                   78L05 regulator

  S1, S2              Pushbutton switches (press to make)
  S3                    8 way DIP switch (Optional - see text)
  Bz1                 Piezo buzzer, passive (See text)
  X1                  11.0592 MHz HC18U crystal

  9V battery holder, PCB
  Nuts, bolts, hookup wire
  6mm smoky grey Perspex for front panel/case

Additional Notes    

While a few years have gone by since this sand timer was built and published, I've found that my prototype sand timer is still being used from time to time around our house, mostly by my (now, much older) children. It's been used to time homework, tests, and even music practice. As a result of this long term use, there are a few thoughts that can be added to the original description (above) which I'll add here.

The original circuit included a pad for Port 1.4 of the 87C751. This was shown on the circuit and PCB but not really discussed further. I have highlighted this with a label 'Output' on the circuit diagram above.

This pin provides an "active low" output when the timer's alarm sounds, and is cancelled (i.e. It goes to the "logic high" state) when the user depresses either button. It could be used to drive a relay or some other device via a suitable buffer transistor if required. The software includes that functionality. I guess I forgot to write that up in the final article because I never used it around here.

The original version did not include any mention of a battery holder. Part of the reason for not including one on the PCB itself was to reduce the PCB's size. Fine, but the sand timer still needed one somewhere, and somehow it was just never mentioned!

Another problem at the time was actually locating one to suit a standard 9V battery. I just couldn't find one anywhere locally. Eventually, several years later, I came across one on a visit to Hong Kong, and I glued it into place on the bottom (horizontal) Perspex surface of the timer. This was a great addition because it made the timer base heavier, and much more stable. It's otherwise a very light little unit.

The addition of the battery holder also required the addition of a power switch to my prototype long after the original design was completed. It's a slide switch removed from an old transistor radio.

The piezo buzzer (Bz1) is a small piezo speaker which came from a musical greeting card. I just unsoldered the small wires from the greeting card circuit board which connect to the speaker and mounted it on some double sided tape to the back of the sand timer. It's still stuck there today, so that tape certainly does the job.

When I built the prototype, I was also fortunate to have found some switches which allowed small paper labels to be inserted into the top of the switch push-button. These labels were printed on a laser printer and can be seen in the photo of the prototype. I've not seen these switches anywhere again, and certainly not at my regular parts suppliers.

The PCB layout shown here is from ELEKTOR and is slightly wider than the prototype. As I noted above, the layout is also slightly different from my original prototype.

The biggest challenge facing the constructor was, and remains, programming the 87C751. These have a UV-erasable ROM memory, and require a special programmer. Details are found in the relevant Intel specifications for the device. I guess if you have a stock of this 87C751 chip, you'll probably have the required programmer gathering dust somewhere too (Mine would be gathering dust too except I also have a stock of 87C552 chips and it programs those as well! Some other projects on this site use those chips.)

Frankly, it's much easier these days to use flash programmed 8051 chips, like the Atmel 89C2051, so feel free to revise the code accordingly. Only very minor changes would be necessary.

The source code (software) for the project can be downloaded from this page (See below) as can the Intel-format HEX file. You may copy the software for personal use and modify the code to suit your requirements but please note that copyright for this project and the software is still retained by Elektor.

Again, my sincere thanks to Elektor for their permission to republish this design on my website.


Source code for the software for this timer
(6k ZIP file)

Intel format HEX file (2k ZIP file)
PCB Layout : This ZIP file contains HPGL PLT format files for the PCB layout and the PCB overlay (40k ZIP file)

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