For KS3 Electronics Technology or GCSE Electronic Products

Circuit/PCB Design by Phil Townshend
Project developed by Simon Nicholls & Phil Townshend


This project is based upon the age old 555 timer, and I'm sure has many forms and incarnations in various schools. I have delivered this project with good results but Simon has developed it a stage further providing enclosed casing and the results he achieved last year from the students was amazing.

This project covers the theme of dental hygene and can be part of a
HEALTHY SCHOOLS project at KS2 and KS3.


A good theme for a project is dental hygene and these projects provide a timed period for the user to clean their teeth for, ensuring thorough cleaning every time. When the timer is switched on, the LED lights up, their will be a very brief beep and the timer starts timing. At the end of the timing period, the buzzer sounds. Based on a standard 555 configuration with a couple of simple modifications:

  • The trigger is from an RC timing network to provide a negative spike. This ensures the timer starts when the power is switched on.
  • The output is used to sink current rather than source it - the output provides a 0v supply rather than +v. While the timer is timing the output is high, thus the buzzer has no potential difference across it. In this way the buzzer will sound when the timing period is over, and the output goes low.

Circuit Diagram

This timer is based around the popular 555 IC wired as a monostable. The output is used as a current sink output, meaning it switches the negative supply to the buzzer. Therefore the buzzer will only sound when the output goes low, at the end of the 555 timing period. As soon as the circuit is switched on, the trigger pin 2, is sent low since the capacitor has no charge and both plates are at 0v. The trigger is released almost immediately as C2 charges up. The 555 begins its timing by sending the output high turning off the buzzer. There may be a brief blip from the buzzer as the circuit is switched on as it takes a brief moment for the IC to respond, but this enables the user to know the circuit is working correctly - so all good. D1 ensures polarity protection from the battery and D2 shows the user the circuit is on.

The circuit board is simple and has the minimum of wiring


The timing period follows the standard 555 formula where C = C3 and R = VR1.

T = 1.1CR

It should be noted however that some capacitors have such large tolerances the period can increase by up to 40% but with the values shown for C and R the time can be varied up to:

Time = 1.1 x 0.0001 x 1000000 = 110 seconds

so nearly 2 minutes. Turning VR1 all the way clockwise for the maximum time.


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