Nuts & Volts - April 2005

Universal Relay Board

The Applications are Limitless, So Get Your Imaginations Into Gear and Have a Go!

your sensor, you can cheat a little by adding a one- to 10-kilohm resistor in series with the sensor, but also add an equal value to the fixed resistor. Try some experimenting with resistor values; you can get quite creative and produce very wide or narrow windows for temperature sensitivity, etc. Just remember sensor current ratings, though. Remember to maintain a 50/50 sensor/resistor ratio for better ±VR1 rotation.

Table 2 shows some examples of thermistor resistance/temperature resistances. Don’t forget to replace C1 after setup.

If you are interested, the relay can be made to self-latch by taking the spare pad between the relay and VR1 and connecting it to one of the relays’ normally open (NO) terminals. Linking pads are provided onboard. The matching “C” common terminal is then taken to the Neg supply rail. By doing this, you effectively bypass the FET Q1 after it has initially engaged the relay. You need to insert a switch in this connection to break the circuit to release the relay and reset it.

The current rating of the left-hand set of relay contacts is only rated at around two amps due to the circuit board

Possible uses could include:

• Battery protection for cars, low voltage accessory cutout

• Battery charger (NmHi, NiCad, SLA) cutout/warning, heat monitoring, overvoltage

• Baby monitor using microphone input

• Garden light controller, sunset activation

• Aquarium/incubator heater control or failure warning

• Power failure warning (battery backup supply needed), use voltage divider input

• Fire warning, connected to security system

• Intruder sensor, use LDR/LED beam, etc., in a security system

• Hand clap activated switch, breaking glass sensor in a security system

• Peltier device control, fridge, or incubator

• Garage door opener (LDR in a tube), set to activate with car high beam lights

• Voice-activated switch for an intercom, CB radio, transmitter, etc.

• Computer automation of other devices using logic voltages

• Toxic gas detection using a metal oxide range of gas detection sensors

track width. The circuit draws just under one mA at 12 volts on standby, 45 mA with the relay and LED on, and about 62 mA if you use the latching option when the FET is bypassed.