From the Technology Enhancement Programme (TEP), these circuits allow students to build a range of devices. Each circuit has a full circuit diagram, component list and explanation of how the circuit works. In this section, the circuits cover a range of timers and timing devices. Many circuits use the 555 timer module.
555 timer (on for a period)
Circuit switches on an LED for a certain period (period LED is on is adjustable).
555 timer (off for a period)
When this circuit is triggered, it will remain off for a certain time. This effect is particularly useful when arming an alarm switch, as it gives the owner time to vacate the house before alarm is armed.
When a push-to-make switch is pressed and then released, a red LED will light after a pre-determined time, (a yellow LED is on continuously).
Transistor (extended timer)
A buzzer sounds approximately five minutes after the circuit has been switched on. Time at which buzzer will sound can be varied.
A buzzer will switch on after a predetermined time and will remain activated. This is known as a latched circuit.
555 touch timer
When sensor pads are touched with a finger, an LED will light for a predetermined time.
When this circuit is switched on the LED will light after a time. The time of switch on can be set using VR1.
555 timer astable
This circuit illustrates the operation of the 555 timer configured in the astable mode, i.e. two LEDs will be made to flash on and off at a predetermined rate.
Once triggered, LEDs will continue to flash until the power is removed from the circuit or a switch is opened. The rate at which LEDs flash is controlled by adjustment of a variable resistor.
Set time on-off
A circuit that demonstrates how the periods that LEDs are lit can be varied.
The 3909 timer
This circuit is used primarily to switch an LED on and off at a predetermined rate.
Steady hand game (timer)
The object of the game is to try to pass a small circle of wire along an erratic loop of wire without touching the loop. In the event of the loop being touched by the ring, a buzzer will sound for a predetermined time.
A seven-segment display which is incremented by one every second.
Long period timer
This circuit switches on an LED after an extended period. The circuit can be modified to switch on any piece of electronic equipment after an extended time period.