PTCL Electronics

Department of Chemistry University of Oxford

Mid complexity circuits include those that have an op-amp or timing chip within them. Chips of this type typically will have several different types of component, due to the fact that different manufacturers produce different ranges.

As far as op-amps go, there are some that are fast and some that are slower, but 1/10th  the price.


This circuit was taken from

The above circuit can be divided into two parts making it easier to digest. The left half is a mono-stable circuit, so when the switch is switched, the circuit will turn on and off the output at a set frequency so long as there is power available. The right half takes this signal (15V pulse) and activates a relay. This is an electromagnetic switch; so when power is applied to the input, the output 2 pins are shorted together. This output pins (in this case) are connected in series with a lamp and an AC source, so the light will light when power is applied to the input.

This circuit will therefore turn the lamp on for a period of time, and then it of again, dependant upon the pot.

This has several useful functions with a minor modification to the output relay, changing it from a single pole single throw, to a single pole double throw;

  • As a corridor light switch to allow a person to walk down the corridor before turning the light off to save energy after 30 seconds,
  • To set a time for a light source to expose a sample in an experiment for 20 minutes,
  • To turn on a unit after a set time, such as turning an oven on in 2 hours time so that it is hot enough for an experiment when you return from a meeting. With some more circuitry such as a second timer between the first timer and the relay, it can turn the oven of again after a period of time such as if the meeting over ran. (This circuit would need us to sort out the second half due to safety requirements when working with mains voltages.)