Water detetor circuit

[adk315]

Not sure if this will work, using a moisture detector circuit to trip a relay that sounds an alarm if water is present in a diesel fuel tank. I will be using this system in a 12 and 24 vdc system with different relay coil voltages. Will this work with both voltages and will 1/4 resistors be ok to use. Please advise.

Adk315

 

[Resqueline]

For the 24V version I'd increase both the 1k and the 2.2k 1/4W resistors to 3.3k in order to ensure keeping their power dissipation within limits under all conditions.
The same resistance can be used for the 12V version. Sensitivity won't suffer noticeably. Remember to put a protective diode across the relay coil just to be safe.

 

[adk315]

Water detector

Resqueline

Do I have the circuit correctly layed out for a water detector, do you see any problems or what would you change, and I am not sure how to use the diode on the relay, is this for voltage spikes?

Thank you

Adk315

 

[Resqueline]

It will certainly work. I'd not expect the water to be so pure that it won't conduct sufficiently. Wether you'll encounter problems or not will depend on circumstances.
If the water droplets turns out to be very pure you might need to exchange one of the transistors with a darlington version. Exchanging both might make it too sensitive.
If someone might use a radio transmitter (cell phone) nearby you might experience false alarms. If that would be bad just add small capacitors between base - emitter.
To learn all about the why's & how's about using diodes on relays just Google relay diode. The relay will generate a spike if the transistor turns off fast.

 

[adk315]

Thank you for your help

Adk315

 

[adk315]

Resqueline
Just one more question
If I want to use the same board for 12 and 24 vdc, can I use a 12 vdc relay coil with two circuits for the coil using a berg jumper, one circuit for 24v uses a resistor across the coil and the other is 12v direct to the coil, if so what would you reccomend for resistance 400ohm?, and can I use a 1N4148 diode at the same time to protect the circuit from voltage spikes?

Thank you

Adk315

 

[Resqueline]

Yes, you can use a resistor in series with the 12V relay to make it operate at 24V. It needs to have the same resistance (& power) as the relay coil you'll be using.
A 1N4148 could be used, but just barely. I'd feel more comfortable with a 1N4002.

 

[adk315]

Resqueline

Thanks again

Adk315