Need an electronic circuit for my project

[pankajbiswas]

Hi all,
In my project I need an electronic circuit which will provide NO contact for 20.3 VDC & it will provide NC contact for 11.7 VDC. Actually I want to control fire detectors through this circuit. In normal condition the detector gives output of 20.3 VDC but when it will be actuated it will gives an output of 11.7 VDC. Now I want to take the detector's signal to the DCS end. As the DCS will only accept the potential free contact,I need to generate potential free contacts in these two different states. I had tried 24 VDC relay to solve this but the problem arises with the reset level of the relay. The relay got set on 16.4 VDC but it got reset around 6 VDC. So the state was not changed though the detector's signal was changed. Please suggest a circuit to solve my problem.I am looking forward for your help.

[pebe]

Here is a circuit that should be OK for you that uses a 555 as a bistable. The zener drops the input voltage by 10V.

The relay switches on when pin6 is above 2/3 of the supply volts, and switches off when at pin2 is below 1/3 of the supply volts. If you want to use a different supply to +12V, then change the zener voltage to suit.

[pankajbiswas]

what will be the zener diode voltage.....if I use a power supply voltage of 24 vdc ?

[pebe]

As the circuit is, a 24V supply would need an 8V swing at pins2 & 6. But you only have 8.6V at source (20.3 to 11.7) so the parameters would be a bit tight.

I'll see if I can modify the circuit for you.

[pebe]

It will be more complicated using a 555 because that won't stand a supply voltage of 24V.

I have drawn a simpler circuit for you. The opamp can be any type - it's not fussy. I have built in a degree of hysteresis so that the relay switches on and off cleanly, but if your input voltage only has two states, 22.3V and 11.7V, then you can leave out R1 and R2 and wire the bottom end of P1 direct to 0V.

The relay switches off when the voltage is higher than 16.5V so take the output from the Common and NO contacts.