Comparator Relay Board using LMV331

This versatile comparator relay board serves as a flexible switching solution for applications ranging from environmental sensing like light and temperature control to critical circuit protection against voltage and current fluctuations.

Light - Power Control January 10, 2026 by mixos

This comparator relay board is a simple and flexible solution that can be configured for a wide range of applications, such as automatic light/dark switching, over-voltage and under-voltage protection, over-current and under-current detection, pressure-based switching, temperature control, and interfacing with various analog sensors.

Note: All resistors and capacitors on the board use SMD 0805 packages. Users may populate only the required components; unused component positions can be left unpopulated.

For 5V applications, the board can be build with the LMV331 comparator in a SOT-23 package. For higher-voltage applications, the TL331 comparator can be used, which supports supply voltages up to 36 V. When operating at 5 V, a 5 V relay is recommended. For higher supply voltages, it is recommended to use a 12 V or 24 V relay.

A trimmer potentiometer is provided to set the reference voltage. The reference can be connected to either the inverting or non-inverting input of the comparator using the provided resistor network. Resistors R6 and R12 are used to configure the upper and lower threshold reference voltages. Resistor R4 can be used to introduce hysteresis and prevent relay chatter near the switching threshold.

Note: The user must select appropriate capacitors, resistors, the correct comparator IC (LMV331 for 5 V or TL331 for higher voltages), and—most importantly—the correct relay based on the application requirements.

Features

5 V DC supply operation
Relay output contact Rating 10A
Trimmer potentiometer for reference voltage adjustment
On-board Relay status LED
On-board Power LED
Screw terminals for relay connections
Header connector for power input
Header connector for sensor or input signal
4 x 4 mm Mounting Holes
PCB Dimensions 47.47 x 29.21 mm

Reference Voltage

The PR1 multi-turn trimmer potentiometer is used to set the reference voltage. Resistor R6 is provided to set the upper threshold, while resistor R12 is used to set the lower threshold. Appropriate resistor values should be selected to define the required upper and lower reference voltages. It is important to consider the allowable input voltage range of the comparator’s inverting and non-inverting input pins when setting these thresholds.

Connections

CN1: Pin 1,2 = VCC, Pin,3 = GND
CN2: Pin 1 = VCC/HIGH, Pin 2,3 Input, PIN 4 = GND/LOW
CN3: PIN 1 = Relay Normally Closed, PIN 2 = Common, PIN 3 = Normally Open
D2: Relay LED
D1: Power LED

Schematic

Parts List

NO	QNTY.	REF.	DESC.	MANUFACTRURER	SUPPLIER	SUPPLIER PART NO
1	2	CN1,CN2	4 PIN MALE HEADER PITCH 2.54MM	WURTH	DIGIKEY	732-5317-ND
2	1	CN3	3 PIN SCREW TERMINAL PITCH 5.08MM	PHOENIX	DIGIKEY	277-1248-ND
3	2	C1,C3	100nF/50V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
4	1	C2	10uF/25V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
5	1	D1	LED RED SMD SIZE 0805	OSRAM	DIGIKEY	475-1278-1-ND
6	1	D2	LED RED SMD SIZE 0805	OSRAM	DIGIKEY	475-1278-1-ND
7	2	R2,R15	10K	YAGEO/MURATA	DIGIKEY
8	1	Q1	BC857AL	ONSEMI	DIGIKEY	BC857ALT1GOSCT-ND
9	1	RL1	5V Relay	AMERICAN ZETTLER	DIGIKEY	3385-AZ943-1CH-5DE-ND
10	4	R1,R6,R8,R12	1K	YAGEO/MURATA	DIGIKEY
11	5	R3,R4,R10,R11,R13	DNP		DIGIKEY
12	4	R5,R7,R9,R14	0E	YAGEO/MURATA	DIGIKEY
13	1	U1	LMV331	TI	DIGIKEY	LMV331M5/NOPB
12	1	PR1	10K TRIMMER POTENTIOMETER	KYOCERA	DIGIKEY	478-601030-ND
13	1	D3	S1MB-13	DIODE INC	DIGIKEY