|The 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 back in June 1995)|
|Here is a microprocessor based version of the original sand and glass type sand timer I designed 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.
|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, since 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 typical 3 minute sand timer was not nearly long enough.
The Electronic Sandtimer 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 of the Electronic Sandtimer. Still in use, the prototype has a slightly different layout to that published by Elektor with buttons on the right hand side and the addition of a slide switch for turning the power on and off on the timer|
|The 87C751 Microcontroller|
|The microcontroller used in the sand timer is one from the 8051 family. Originally developed by Intel, the device is 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.
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 external 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 sand of a 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.
|How It Works|
|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 micro directly drives all of the display, control and alarm interface. 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 in fact serves a triple duty. Its prime role is to emulate the sand in a sand timer, of course. Its secondary duty is to provide 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. The LEDs are then used to display the alarm time from 1 to 99 minutes, displaying the incremented time as the SetTime button is held down.
The display consists of only 17 LEDs, and 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 used, showing the time briefly before reverting to the random display.
|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 strobing 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. It 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 allow fast operations, 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 indicates that the time has elapsed by flashing all the LEDs, and by emitting a warble from the piezo speaker. The use of the piezo speaker maximises the volume while minimising current consumption. This avoids the alternative, the use of a driver transistor and 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 of the printed circuit board is shown below. (Note: This is the Elektor PCB layout and differs 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 NPNtransistors. (BC547B) and six PNP transistors (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 opted to mount the LEDs and pushbuttons about 6mm off the PCB. This was done to allow for the height of the microprocessor and socket which would otherwise prevent the buttons from sitting correctly through the front panel. My prototype had the LEDs down hard onto the PCB, as were the switches. My switches were high enough to be pressed through the front panel but smaller switches may need to be mounted slightly higher.
Also, the overlay and parts list note the transistors as T1-T9 while the circuit diagram shows these as Q1 - Q9.
|The sand timer 'wakes up' set for ten minutes but the DIP switch and diodes allows other 'wake-up' times to be set. The DIP switch (S3) settings are shown in this table and 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. 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 only has 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 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 at power on only. 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 chip 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 greater 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 starts. 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 photographs. 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 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.
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
|While a few years have gone by since this sand timer was built and published, I've found that the original 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 and tests, and even music practice. There are some additional thoughts that could 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 discussed further. I have highlighted this with a label 'Output' on the circuit diagram above. In fact, this pin provides an active low output when the timer's alarm sounds, and is cancelled when the user depresses either button. It might 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, but somehow it was just never mentioned! Another problem at the time was locating one for a 9V battery. I 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 very light.
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 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 since that time at my regular parts suppliers.
The PCB layout shown here is from ELEKTOR and slightly wider than the 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 the chip, you have the required programmer gathering dust somewhere (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 minor changes would be necessary.
The source software for the project can be downloaded from this page (See below) as can the Intel 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 retained by Elektor.
Again, my sincere thanks to Elektor for their permission to republish this design on my website.
|Intel format HEX file|
|Assembled LIST file|
|(Just in case someone wants it for some reason.....)|