IO goes high just by touching it

[ngmedaboard]

I have an IO on a microcontroller tied to a transistor that drives a relay. It doesn't seem to be operating properly and I notice if I touch the IO with my finger it goes high or at least high enough to turn the LED on. I am not closing the circuit to anything but simply touching the IO.

Is something capacitively coupling somehow using my finger?

-NGM

 

[thylacine1975]

Aye, it sounds like your IO is configured as an input - the voltage on your finger (due to your body acting as an antenna) is sufficient to bias the subsequent transistor into (partial) conduction.
Check your IO direction settings

 

[D.A.(Tony)Stewart]

inserting stay hum is a good test that your drive level is marginal and your circuit is sensitive to stray common mode noise. ( aka EMI ingress)

Check Vcc for transients using a scope with very short ground leads.
Check Coil voltage with two probes in diff mode A-B with short ground to see what the output looks like.

If you happen to be powered by a coin cell that would explain why, then add a sufficiently large cap across the Vcc.
If that fails and your transistor base resistor may be far too high to saturate the switch.... Note most transistor specs for switch use ( except Zetex*) use Ic/Ib=10 That means ignore hFE when in Vce is in saturated mode.

I might add that few design Engineers today realize that not only do transistor switches have a low gain of 10 at rated Vce sat. but Relays too have an effective current gain of 10 when you compare DC max contact current to DC coil current. I wish Universities would teach the benefits of modern automotive Integrated switches. low voltage low current logic level drive . > 2A drive, very low Ron and very cheap. << $1

hint search for "High Side or "Low Side" IC switches when you get a chance. Digikey have a ton of them. PMIC - Power Distribution Switches (1854 items)

 

[wp100]

Hi,

What happens if you put a 10k resistor from the transistors base to 0v.