Hi.All.
I am looking for a circuit to monitor a variable voltage up to 1VRMS and trip a relay if the voltage is reached, a circuit,a pre built module that I can adapt or any other ideas,,,can anyone help.
Geoff (newbe to this site)

 

Use a comparator IC (op-amp) with its inverting input at a reference voltage of approximately 1.41 Volts and apply your signal on the non-inverting input. When the signal will reach 1 Volt RMS (1.41 V peak) the comparator will output a positive voltage and activate your relay.

 

Thank you MIK3, is there any circuit and component ref. around, took HNC back in 74 now retired and out of step with modern components.
Regards Geoff.

 

You might look at an LM2903 dual comparator. Low power, can be used w/single or dual supply, common mode includes input down to the negative rail, and it's suitable for automotive/industrial temp ranges.

 

Hi.All.
I am looking for a circuit to monitor a variable voltage up to 1VRMS and trip a relay if the voltage is reached, a circuit,a pre built module that I can adapt or any other ideas,,,can anyone help.
Geoff (newbe to this site)

HI
I AM NOT AN EXPERT IN ELECTRONIC'S
BUT YOU CAN USE OP-AMP IC'S LIKE (LM339N)
ASK FOR THE COMPLEAT CKT.
GOOD LUCK
AND Mr MIK3 IS RIGHT
MKBUTAN.
∏∏∏∏∏∏∏∏∏∏∏√

 

An LM339 is a comparator.

I strongly recommend against using an opamp for a comparator function, as the opamp's output stage will be in constant saturation due to being operated in open-loop mode; this means high power consumption and the likelyhood of a short life span.

Comparators generally have open collector outputs. This means that they can sink current, but cannot source it. If you need a current source, then you can add a pull-up resistor.

To calculate the value of pull-up resistor:
1) Look at the datasheet to determine the minimum sink current. For this example, we'll use 6mA, and Vcc=12v.
2) divide the minimum sink current by 2. (6mA/2 = 3mA)
3) Divide Vcc by the result to get the resistance. (12v/3mA = 12/0.003 = 4k Ohms

 

See if this will do. Remember that the USB port is limited as to current, so use a relay that won't overdraw the port.

Get the data sheet for the LM311 and read the various gotcha's about them. The R2 resistor is a guess about hysteresis - you will need some if your monitored voltage changes slowly.