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555 (TLC555) Relay Driver Circuit

Many integrated circuits have undocumented features or abilities. This is one of them. The TLC555 output (pin 3) can sink a 100mA load to 1.28V. The open drain transistor reset (pin 7) can sink 100mA to 1V. Tying both lines together is permissible because they are logically the same polarity and this potentially doubles the sink current ability to 200mA. This is ideal for driving my 133mA relay coil.
The input also has the undocumented, but better known, feature of a Schmitt trigger that provides positive switching.

555 Relay Driver Schematic

TLC555 Relay Driver Schematic

Protoboard setup

Output current rating (pin 3): TLC555 vs. NE555

(Vcc = 15V) TLC555 NE555
Source current 10mA 200mA
Source mode saturation voltage Vcc-0.8V Vcc-2.5V
Sink current 100mA 200mA
Sink mode saturation voltage 1.28V 2.5V

While it can be seen that the NE555 has the higher current rating, its saturation voltage is grossly inferior and this is a detriment in driving loads without excessive voltage drop. Also it can be seen that the TLC555 is much like TTL in that its sourcing ability is far less than its sinking ability. However for driving a relay, we are interested only in current sinking properties.

Relay out of my junk box

12V Contactor

This is a 40A automotive relay (contactor) that I selected for this application. Since its coil current exceeds the TLC555 output sink current rating, it is a good candidate. Manufacturer or part number is unknown.

  • Contact rating: 40A (from memory)
  • Coil resistance: 90Ω cold
  • Coil current: 133mA
  • Coil power: 1.6W
  • Coil voltage: 12V
  • Pickup voltage: 4.8V cold
  • Dropout voltage: 2.1V cold
  • Operate time: 4.5mS

What is the difference between a relay and a contactor? There is no clear difference other than perhaps current rating and/or application—to me, relays are anything from signal devices to approx 20A. Anything rated at 40A or larger, or is used in power applications is a contactor.

Schmitt trigger input

The trigger input (pin 2) and the threshold input (pin 6) pins are tied together—this is commonly done. With a 12V supply, the upper threshold is 8V and the lower is 4V. The two voltage levels, being far apart, make a great Schmitt trigger. This may be driven directly by 4000series CMOS logic that is also powered via 12V.

To make it compatible with TTL logic levels, simply add the two resistors that are shown in the schematic. This loads down the internal divider to a lower voltage. The calculated levels are approx. 1.4 and 2.8V respectively.

Testing the performance

  • Output saturation voltage:
    • 0.617V @ 133mA
    • 1.043V @ 200mA
  • TTL high level threshold: 3.0V
  • TTL low level threshold: 1.6V

How about the bipolar 555 and other variants?

The standard 555 saturates poorly (2.5V) at 200mA and its reset line (pin 7) is not specified above 15mA, so it is not recommended. The intersil 7555 is a very low power device, so that is not recommended either.

For the future

  • Assorted relay driver circuits
  • 555 Voltage doubler relay driver circuit
  • Relay economy circuits

6 Comments

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  • Kuberkoos

    In my neck of the woods a contactor is used to drive a motor, heater element or lighting circuit bigger than 1kW. Anything smaller than this is driven by a relay. This is,however, most probably a rule laid down by the company I worked for as an electrical draughtsman many years ago!

  • Sparkletron

    I managed to do this via the internal latch provision–but only by adding a transistor to raise the output current from 200ma to 400ma needed for my relay. I couldn’t get your pin 7 trick to work. When I tie it to the output, it does indeed raise the current to the desired amperage–but it also reverses the normally low output. I don’t want the relay coils to stay energized waiting for an event. BTW, I’m using a regular 555 hence the greater currents.

    • Jim Keith

      My guess is that there is something else happening to cause the output voltage inversion. One issue that has not been covered is how to preset the internal latch of the 555 because when it wakes up (power is applied), the latch may be in either state.

  • Sparkletron

    Thank you for this great post. How would you modify this circuit so that once the relay is engaged, no amount of change on the part of the trigger input will disengage it? I’m trying to make an alarm, and once it fires I don’t want it to stop.

    • Jim Keith

      The easiest way to do this is to use a 2 pole relay –use the 2nd pole as a holding contact to keep the relay picked up –connect the normally open contact between the low side of the relay and common. Reset the relay via a normally closed contact in series with the holding contact or simply recycle the power.

      This can also be done via the internal latch provision of the 555. If you want to try this solution, let me know and I will post a new circuit.

  • John

    I have a vague memory that a relay uses a low voltage coil to switch a high voltage load, whereas a contactor coil uses the same voltage as the switched load as in a car – a small 1A 12V dash switch operates a much larger 40A 12V contactor in the engine bay.
    Also the contactor is manually switched, while the relay is automated.
    (Old UK telephone exchange training – GPO)

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