Repeating Timer 5

  
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This timer circuit will only run when the temperature rises above a preset level. There`s an alternative version - Repeating Timer No. 6 - that only works while the temperature is below the preset level. R7 lets you set the temperature. The value of the thermistor is not critical. The important thing is the voltage on pin 1. Any value thermistor sh
Repeating Timer 5 - schematic

ould work satisfactorily. But you may need to change the value of R7 - to achieve the desired range of adjustment. The circuit is basically an Astable Oscillator. And the output times depend on the value of C1 - and the speed at which it charges and discharges through the resistor network. The length of time the relay remains energized is controlled by R1 & R2. And the length of the time it remains de-energized is controlled by R3 & R4. The fixed resistors set the minimum period lengths - and the maximum period lengths are set by R2 & R3. With the component values shown - both periods are adjustable from about 1 to 30 minutes. You can change the component values to suit your own requirements. If your time periods don`t need to be too precise - and more-or-less is close enough - you can leave out the pots altogether - and simply rely on R1 & R4 to set the times. Owing to manufacturing tolerances - the precise length of the time periods available depends on the characteristics of the actual components you`ve used - and a 4093 will produce longer time periods than a 4011. Do not use the "on-board" relay to switch mains voltage. The board`s layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage - mount a suitably rated relay somewhere safe - Away From The Board. I`ve used a SPCO/SPDT relay - but you can use a multi-pole relay if it suits your...



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