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.
Output current rating (pin 3): TLC555 vs. NE555
|(Vcc = 15V)||TLC555||NE555|
|Source mode saturation voltage||Vcc-0.8V||Vcc-2.5V|
|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
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.
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
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