What does it do?
| The delay subsystem produces a delay after the input signal goes high. |
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How does it operate?
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Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
The timing period begins after the input signal has returned from high to low.
If the input signal changes from low to high during the timing period, the timing starts again.
The delay circuit uses the ability of an electrolytic capacitor to store charge.
The two inverters sharpen up the input signal. A high input signal switches on the transistor and causes the capacitor to discharge. During this period, the output signal is forced high by the right-most inverter.
Timing begins as soon as the input signal goes low. The transistor switches off causing the capacitor to charge through the variable resistor. |
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Once the capacitor has charged to about 50%, the right-most inverter will trigger the output back to low.
The maximum length of the delay can be calculated:
Delay time (t) = 0.7 ´ C where C is the capacitance in mF.
So, if C = 10mF, the maximum delay is 0.7 ´ 10 = 7s.
If a longer delay time is needed then a larger capacitor can be used. |
Possible applications
- Making a buzzer sound for a fixed length of time after a switch has been pressed.
Making
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Pins of 4069B IC
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How part of the PCB might look
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The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.
The transistor used on the PCB is the BC337, but any low power NPN transistor e.g. the BC547B, can be used.
Build and test the unit that will provide the input signal before building the delay unit.
Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:
- the voltage on pin 7 is low (0V);
- the voltage on pin 14 is high (the supply voltage);
- the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.
Insert the IC the right way round.
Testing
Turn the variable resistor to its mid-point. Make sure that the signal going out (on the green PCB track – from pin 6) changes from high to low.
Fault finding
If there is a fault, check that:
- The voltage on pin 7 is low (0V)
- The voltage on pin 14 is high (the supply voltage)
If there is a fault, check the tracks and solder joints.
Alternatives
- 555 monostable – more accurate timing but no ‘spare’ gates
- The retriggerable monostable provides a pair of simple, acurate delay units but are more expensive.
- PICs – more flexible and accurate but more expensive.
Web links
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