Hey guys, nubey here, or more nubey than I want to be. I need help on a project where I'm looking to activate a relay when resistance is within a certain range. Is there anything out there I can buy? I looked in all the normal places but can't find anything. Resistance can't be a wide range and will need to be around 6k. Thanks for the help...

 

When What resistance is within a certain range?
A little more explanation of what you are trying to do would help??

 

Sure can do.
Use the resistor in a voltage divider configuration (or bridge circuit) and apply the voltage difference to an analog comparator.

 

What is generating this resistance and what is its range?

 

There is a circuit that is easily adapted to monitor a resistance. It is called a "window comparator. You will need to provide asupply to pass a current through the resistance so that you have a voltage to be monitored. It is cheap and easy to build and use. I am sure that such modules are available, the circuit is published in many places. But I would not look for it on yoo toob.

 

What is generating this resistance and what is its range?

I have a resistor in an ice maker that I want to cycle the ice when it reaches 20K ohms. When warm water enters the ice maker the resistance goes down to 5k and as it cools down and freezes the resistance climbs. At 20k ohms I want it to close a relay.

 

You could probably do it with a LM311 comparator

 

It sounds like you have a thermistor, not a simple resistor. What is the power source you have available for a module or circuit?

So, you have a thermistor, and you want a relay to close when the resistance increases above 20 K, and open when the resistance decreases below 5 K. Correct?

If yes, this seems backwards. Wouldn't you want the relay to open (remove power) when things are cold enough?

Separate from that, there are a ton of mechanical and electronic thermostat devices / parts / modules on ebay.

ak

 

It sounds like you have a thermistor, not a simple resistor. What is the power source you have available for a module or circuit?

So, you have a thermistor, and you want a relay to close when the resistance increases above 20 K, and open when the resistance decreases below 5 K. Correct?

If yes, this seems backwards. Wouldn't you want the relay to open (remove power) when things are cold enough?

Separate from that, there are a ton of mechanical and electronic thermostat devices / parts / modules on ebay.

ak

You are correct, what i meant is thermistor. Not sure why Im saying resistor. I need to apply power at 20k using a reversable polarity timed relay. anything under 15k would work to turn it off again.

 

Then the Comparator circuit should work, is there access to low voltage DC?

 

Copied from post #3.



R3 + Rsensor constitute a voltage divider.

If the excitation voltage is 12V, for example, setting R3 = Rsensor = 20kΩ will give 6V at the junction.

R1 + R2 is your reference voltage divider.
If R1 = 20kΩ and R2 = 15kΩ, your reference voltage is 12V x 15k/35k = 5.14V

Thus what you need is a window comparator (using two comparators). Or use a single comparator with hysteresis.

When Vout exceeds (6 - 5.14)V = 0.86V the relay must turn on.
When Vout falls below 0V the relay turns off.

Adjust the values of R1, R2, and R3 to give you suitable switching thresholds.

 

You are correct, what i meant is thermistor. Not sure why Im saying resistor. I need to apply power at 20k using a reversable polarity timed relay. anything under 15k would work to turn it off again.

That's a NTC thermistor , so a simple comparator circuit will work with a LM311 op amp.

 

Then the Comparator circuit should work, is there access to low voltage DC?

Sure, i can do whatever i need

 

Thermistor sensor, comparator, and adjustment, $0.99 with free shipping.

https://www.ebay.com/itm/3144917207...gXT+3ij9SueOT64VUIluD7SA==|tkp:Bk9SR4j9qqyFYg

ak

 

Sounds like a job for a 555.

 

Is this not a thermometer with a relay output ?
such as , ( one of VERY many, just have a hunt around )
https://www.ebay.co.uk/itm/185853370816

 

I suggested a "window comparator in post #5. There may be an assembled one available as a module someplace. Having the name of the device makes the search easier.

 

I suggested a "window comparator in post #5. There may be an assembled one available as a module someplace. Having the name of the device makes the search easier.

Post# 14 ...

 

Post# 14 ...

Also #7 !

 

Also #7 !

Note that all of the information that comes with that module in the link is equal to the information provided in post #15.
The item shown in the link on post #16 might possibly be useful. Maybe.