Article : Andy Collinson
A simple battery operated one hour timer device with an audible warning. May be used as a parking meter timer.
This circuit uses just two CMOS IC's, a 4011 quad 2 input NAND gate, and a 4020 14-stage ripple binary counter. At switch on R2 and C2 provide a brief reset pulse, which will ensure the output pin Q1 of the 4020 is high. Gates U1 and U2 form a simple astable R1 and C1 determining the timing period. The tolerances of capacitors vary widely, so for more control, you may use a 470n capacitor for C1 and use a fixed 3.3M resistor in series with a 250k preset for R1.
A timing period of just less than 1.76seconds is required.
The output of the oscillator at U2 drives the input of the 14-stage ripple counter, U3. The outputs divide sequentially by two and the output signal is taken from Q13, requiring 2048 input pulses before the signal becomes high.
When the output Q13 goes high, the output sounder will become active. Gate U4 of the 4011 is used to "modulate" the output sounder. As U4 is also connected to the output of U2, the output sounder will turn on and off at the same rate as the oscillator.
Suitable output sounders can be found at Maplin Electronics part code KU56L or CR34M. These are self contained DC piezo buzzers, requiring 10mA at 12V DC but work with supply voltages from 3 to 15 Volts DC.
A downloadable simulation version is available here
. This has been tested with LT Spice version IV. From version 4.05 onwards, transient simulation can take a long time.
To speed up simulation, I have set initial conditions on the circuit node connected to gate U1 to zero. This speeds up simulation time to a matter of seconds.
LT Spice is a freely available schematic capture and spice simulator program made by Linear Technology. The link to their software page is here.
There is also an active yahoo group available for LT Spice which you can find here
The graph below is from the simulation version of this circuit. In the simulated version I have tapped the output of the CMOS4020 at Q5, therefore only 8 input pulses from the oscillator (shown in green trace) are required before the
Q5 output switches to high (shown as blue trace). The top waveform in red, is the output across the output sounder.
As can be seen, this output is switched on and off as long as the output pin, Q5 is active. To simulate the sounder,
I have used a fixed resistor.
Here comes the maths. One hour or 3600 seconds divided by 2048 pulses (Q13) requires a timed period of 1.7578 seconds.
The timing for a CMOS oscillator, varies with supply voltage, but is approximately 1.1 RC. To achieve the timed period,
C1 is 0.47u and R1 is made from a fixed 3.3M resistor in series with a 250k preset.
To adjust this value, connect a low current LED and fixed 2.2k resistor to the output of IC2. The LED should illuminate on each pulse. Adjust the 250k preset
until the LED flashes about 34 times per minute (60/34 = 1.76s). If you would like to use this a parking meter timer, then set the unit to trigger before the
hour is up or start the timer before you feed the meter to allow extra time.