I’m controlling a cheap relay module, intended to run at 5V. It’s hard to trace out the board, but it contains…
- A S8550 transistor (marked 2TY)
- Three 1K resistors (marked 102)
- A red and a green LED (red is power on, green is coil active)
- A 5V relay (SRD-05VDC-SL-C)
- Pin headers for 5V, Gnd, and ‘In’ (which must actually be pulled to ground to trigger the relay)
- A diode of some kind
There’s about 1k between the transistor Base and the the ‘In’ pin header (matches a 102 resistor). Vcc to Emitter is a short, Gnd to Collector ~73ohms (matches coil resistance).
3.3V does seem to be enough to hold the transistor ‘closed’, although it bounces initially. Pulling the ‘In’ pin low through D3, would trigger the module, but would sink (measured) 4.1mA at -5V.
I guess we’re saying these pins are -5V tolerant at that sort of current level?
Having actually tried it though, the bounce I get when applying 3.3V high doesn’t fill me with confidence, so I’ll probably ‘do it properly’ with a NPN transistor