and if so it provides a switching signal to the buffer 15 since the multiplexer 21 and the microprocessor 14 are timed by the advanced zero crossing detector 20 which provides its signal just in advance of each zero crossing point the trigger signal is available at the buffer 15 just before the supply voltage falls to zero at the instant of zero the zero crossing detector 16 supplies a gating signal to the buffer 15 triggering the triac at the moment when the voltage has fallen to zero and so avoiding a voltage pulse which could cause radio interference operation continues in this way the triac 12 being triggered or not as the case may be on each cycle under the control of the microprocessor 14 to maintain the required time temperature relationship a display 22 making use of light emitting diodes may be used to indicate the stage in the process currently reached the microprocessor itself and the timing display driving triac firing interrogation of the controls and the temperature comparators may all be performed by a signal chip for example a cop 9411 two further figures fig4 a and 4b illustrate a preferred control program in flow chart form the control program monitors external conditions through its input parts acts accordingly and displays its current state via its output ports the run loop which monitors the temperature levels fires the triac and inspects the stop pause switch the pause loop which monitors the resume 1 pause switch with the led display indicating the state of the program thus timing is taken from the mains zero crossing being a change in logic level corresponding to mains.