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TCR-102 TTL Relay example of usage

TTL signal connected to the relay controlling a light bulb

In some situations it may be required to connect a relay directly to a TTL logic level signal. This may be the case when some existing board or equipment, without load driving capabilities is needed to drive big loads.


TTL signals have low driving capabilities and are not able to drive the relay coil itself. A typical relay coil require between 40 and 100mA coil current. The TTL outputs from micro-controllers can typically be loaded by 10mA. Some are able to be loaded up to 20mA, however these are mostly designed to control a LED.


The illustration below shows how a board can be connected to the TCR-102 TTL Relay module.

In this example we have a controller board required to switch a light bulb. Obviously, a relay is required to switch the main current. In this case, the controller board does not have any output lines capable of driving a relay itself, so the TCR-102 is the choice of TTL relay.


The red and black wires are 12V DC supply for both the controller board and the relay. The blue line is the signal given to the relay to switch power. And the brown lines are AC current carrying wires. NC = normally connected and NO = normally open. When the relay is switched on by the TTL input signal (high), the current is between COM and NO, and the associated yellow LED is lit.

TCR.102 are ordered here

TCR-102 TTL Relay details

TTL Relay signal input


The relays are controlled by input 3 and 4. These inputs are TTL compatible.


The following voltage levels apply:


Input Low: 0 – 1.0 V = Relay connected COM-NC (LED off)

Input High: 1.8 V – 15V = Relay connected COM-NO (LED on)


The signal input accept voltage up to maximum 15V, meaning the relay can be controlled by other type of signal sources as well.



LED indicators


The TCR-102 has 3 LEDs indicating power status and operating status.

The green LED indicates power is connected and on.

The yellow LEDs indicate relay operation. LED on indicates TTL input signal is high and relay is switched to COM-NO.

Low power consumption

The TCR-102-series TTL relay are designed for low power consumption.

To minimize the power consumption, the LEDs can be disabled. This is done by removing the jumper switches (as seen in the picture above). The power on LED and relay LEDs can be disabled individually. By removing the left jumper switch the power on LED is disabled. By removing the right jumper, the two relay LEDs are disabled.

When the LEDs are disabled, the module consumes less then 20uA when relays are not operated (assumed room temperature.)

Each LED consume 20mA when turned on. The relays itself consume each 400mW. (Current depends on the voltage version, calculated by current = watt / voltage). The LED jumper switches make the module current consumption flexible to fit the available power.


Input protection for professional usage

The internal electronics are protected for common electrostatic discharge by TVS diodes at all signal and power input. A TVS Diode is a clamping device that limits harmful voltage spikes. (The TVS are located on the bottom side of the board.)

No TVS diodes are mounted on the relay connectors, as these lines are physically isolated from the sensitive electronics.


TCR-102 are ordered here