3.3v output for more than one pin at a time

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1

I have noticed that if more than one pin is set to output-HIGH, the most recent pin output request provides 3.3v out while the others are sacraficed (1.5v, .5v, etc). I am using the Adafruit_BBIO.GPIO library to control the pins. I am building a “climate control” system, and using the BBB and a DHT22 temperature/humidity sensor to operate relays to supply 120vAC to components (lights, heater, water pump).

I have: a door switch SPDT(P9_11) for safety override, a DPDT(P8_10 and P8_12) for automatic and manual operation, and a 3-24vDC/0-240vAC relay for suppling power to a lamp.

The P8_12 and P9_11 always have power. If the door switch is activated(P9_11) it activates the relay and bypasses the DPDT switch. If the DPDT switch is set to “manual”(P8_12) the relay is activated. Otherwise if the temperature from the DHT22 is above or below a set limit, and the DPDT switch is in “automatic”(P8_10), the relay is activated accordingly.

Pseudo code:
import libraries

Set pins P8_12 and P9_11 to output 3.3v

GPIO.output(P8_12,HIGH)
GPIO.output(P9_11,HIGH)

while 1:
read temperature

Decide to turn P8_10 on/off based on temperature

if temp<28:
GPIO.output(P8_10,HIGH)
else:
GPIO.output(P8_10,LOW)

Loop indefinately

Here is the problem:
If I run a test program with only the door switch the light turn on/off as expected. If I execute the above code the DPDT switch and auto/manual control works as expected however the door switch will not supply enough power to activate the relay. When the above code is running and P8_10 turn on, or if the DPDT switch is set to manual(P8_12) the voltage across the door switch(P9_11) drops to ~1.5v and is less than the required 3v to activate the relay.

Question:
Can the BBB supply 3.3v out for more than one pin simultaneously, or will i need to add an amplifier circuit to each output?

I can attach full code and schematic if it would be helpful. In the end there will be 8 relays, of which up to 5 will be activated simultaneously.

2

how are you driving the relays ?

i hope you are using a relay driver IC

3

As Wulf man said: Which relay driver are you using?

What happens if you detach the relay driver and sense the outputs directly with a multimeter?
Most GPIO are capable of currents between 4mA and 8mA (see buffer strength table 4.1. am3352.pdf).
Drawing more current can kill the µC !!

Regards
Chilli

4

One other thing not related to your problem (don’t know if you fixed it in the real code)
According your pseudo code your output P8-10 will behave stange when the temperature is almost 28 and the ADC toggles between 28 and the next lower value.
To avoid it you should always use a hysteresis:

const HYSTERSIS = 0.2

if temp < 28-HYSTERESIS
GPIO.output(P8_10,HIGH)
if temp >= 28
GPIO.output(P8_10,LOW)

It depends on your system which hysteresis makes sense.

Chilli