Need an electronic circuit for my project

[Pankaj Biswas]

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.

 

[picbits]

Do a search on comparators - relatively easy to use.

 

[MikeMl]

So would you be happy with a circuit that pulls in the relay when the voltage is greater than (20.3+11.7)/2 => 16.0V?

How much current can the detector send?

Something like this?

 

[eTech]

So would you be happy with a circuit that pulls in the relay when the voltage is greater than (20.3+11.7)/2 => 16.0V?

How much current can the detector send?

Something like this?

Hi

What's the zener diode do?

eT

 

[MikeMl]

What's the zener diode do?

As the voltage at Det is increased, no current will flow into the base of the NPN transistor until it exceeds 16V( Zener conduction voltage) plus 0.6V (forward-biased base-to-emitter voltage). Once the base current exceeds about 100mA (relay pull-in current) divided by the Beta of the transistor (>50 for the one I specified), or 2mA, the relay will pull-in. As the Det voltage goes even higher, the transistor conduction will move toward saturation.

 

[eTech]

As the voltage at Det is increased, no current will flow into the base of the NPN transistor until it exceeds 16V( Zener conduction voltage) plus 0.6V (forward-biased base-to-emitter voltage). Once the base current exceeds about 100mA (relay pull-in current) divided by the Beta of the transistor (>50 for the one I specified), or 2mA, the relay will pull-in. As the Det voltage goes even higher, the transistor conduction will move toward saturation.

Oh..ok

But I wonder if the circuit should be fail safe? That is, maybe the relay should be energized until fire is detected?

eT