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I've got a small 5v relay board ("Ontengfei" JQC-3FF-S-Z) connected to my Raspberry Pi 3 B+. When I set the GPIO pin mode to OUTPUT, the relay turns on. When I toggle the GPIO pin between HIGH / LOW, the relay just stays on, it doesn't toggle like I'd expect it to. If I set the GPIO mode to INPUT, the relay turns off. This is really confusing, because I'd expect toggling LOW / HIGH, while the GPIO pin is in OUTPUT mode, to turn the relay on/off.

Relay image

I'm using physical pin 12, wPi 1, BCM 18, depending on your preference. Even though voltage is set to 0 (LOW), the relay is still receiving a signal.

$ gpio readall

+-----+-----+---------+------+---+---Pi 3+--+---+------+---------+-----+-----+ | BCM | wPi | Name | Mode | V | Physical | V | Mode | Name | wPi | BCM | +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+ | | | 3.3v | | | 1 || 2 | | | 5v | | | | 2 | 8 | SDA.1 | OUT | 0 | 3 || 4 | | | 5v | | | | 3 | 9 | SCL.1 | IN | 1 | 5 || 6 | | | 0v | | | | 4 | 7 | GPIO. 7 | IN | 1 | 7 || 8 | 0 | OUT | TxD | 15 | 14 | | | | 0v | | | 9 || 10 | 1 | IN | RxD | 16 | 15 | | 17 | 0 | GPIO. 0 | IN | 0 | 11 || 12 | 0 | OUT | GPIO. 1 | 1 | 18 | | 27 | 2 | GPIO. 2 | IN | 0 | 13 || 14 | | | 0v | | | | 22 | 3 | GPIO. 3 | IN | 0 | 15 || 16 | 0 | IN | GPIO. 4 | 4 | 23 | | | | 3.3v | | | 17 || 18 | 0 | IN | GPIO. 5 | 5 | 24 | | 10 | 12 | MOSI | IN | 0 | 19 || 20 | | | 0v | | | | 9 | 13 | MISO | IN | 0 | 21 || 22 | 0 | IN | GPIO. 6 | 6 | 25 | | 11 | 14 | SCLK | OUT | 0 | 23 || 24 | 1 | IN | CE0 | 10 | 8 | | | | 0v | | | 25 || 26 | 1 | IN | CE1 | 11 | 7 | | 0 | 30 | SDA.0 | IN | 1 | 27 || 28 | 1 | IN | SCL.0 | 31 | 1 | | 5 | 21 | GPIO.21 | IN | 1 | 29 || 30 | | | 0v | | | | 6 | 22 | GPIO.22 | IN | 1 | 31 || 32 | 0 | IN | GPIO.26 | 26 | 12 | | 13 | 23 | GPIO.23 | IN | 0 | 33 || 34 | | | 0v | | | | 19 | 24 | GPIO.24 | IN | 0 | 35 || 36 | 0 | IN | GPIO.27 | 27 | 16 | | 26 | 25 | GPIO.25 | IN | 0 | 37 || 38 | 0 | IN | GPIO.28 | 28 | 20 | | | | 0v | | | 39 || 40 | 0 | IN | GPIO.29 | 29 | 21 | +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+ | BCM | wPi | Name | Mode | V | Physical | V | Mode | Name | wPi | BCM | +-----+-----+---------+------+---+---Pi 3+--+---+------+---------+-----+-----+

What I've tried

  • Adding various resistors in between the GPIO pin and the relay, to reduce the voltage
  • Tried physical pin 8 and 12; both exhibited the same behavior
  • Added a diode to prevent voltage from being sent from the relay board to the Raspberry Pi. It still worked as above, but didn't fix the issue.
  • Connecting it to 3v3 power instead of 5v (same behavior)
Trevor Sullivan
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    Without detail of the mystery "relay board" who knows? Many (poorly designed) Arduino boards don't work with the Pi. – Milliways Nov 18 '18 at 03:41
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    I'd guess the relay is `Active Low` because both 0v and 3.3v are not 5v (in my experience, 3v3 sometimes "behaves" like HIGH, and sometimes LOW on a 5v relay - depending on a bunch of factors) - the only flaw to my assumption is that setting the GPIO as input turns the relay off (if it's active LOW as I suspect, then the relay would turn off when the input is "HIGH") – Jaromanda X Nov 18 '18 at 22:45
  • `When I toggle the GPIO pin between HIGH / LOW` .... how do you know that is what happens? .... the output may already be LOW – jsotola Nov 19 '18 at 03:09
  • Possible duplicate of [16 channel relay module does not respond to write 0/1 (GPIO.HIGH/LOW) commands](https://raspberrypi.stackexchange.com/questions/64322/16-channel-relay-module-does-not-respond-to-write-0-1-gpio-high-low-commands) – Dmitry Grigoryev Nov 19 '18 at 13:19
  • @goldilocks no, I meant input to the relay – Jaromanda X Nov 19 '18 at 20:26
  • @JaromandaX Makes sense -- my bad, I'll delete all that. Thanks. – goldilocks Nov 19 '18 at 23:34

2 Answers2

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Without knowing the nature and needs of the mistery relay board is hard to help. Nonetheless I'll try. This are steps that I'd try to solve the problem:

  • Connect led diode trough some resistor and see if that lights up as expected

  • figure out what drives the relay (low or high) signal

  • once established that figure out at what voltage relay triggers

  • for RPi protection I'd use optocoupler triggering for relays

  • Try some other GPIO like 24 or 23 as they don't have alternative functions

Bungee75
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  • Thanks, your answer was helpful. It turns out that a low signal actually triggers the relay, not a high signal. I'm still learning. However, the real problem is that 5v was apparently too much for the relay. Connecting it to 3v3 power works just fine. I'm using the `gpio toggle 1` command to switch it on and off. It's working perfectly. – Trevor Sullivan Nov 21 '18 at 18:50
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Even though the relay clearly says it's a 5v relay, it only worked when I connected it to the 3v3 power rail.

Trevor Sullivan
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