Touch Sensor Problems

[seefeldt81]

i'm trying to put together a extremely simple touch sensor. I have made a diagram of what I put together so far.

I'm using a extremely sensitive transistor ( BC549 ) that reacts if you touch its base with your finger. attached to it is a 5v relay and I'm using 5v USB for power.

when I set it up on the bread board I hear the relay clicking when I touch the ( BC549 ) transistor base with my finger but it doesn't seem to fully engage the relay.

**broken link removed**

thank you for your time

 

[Hero999]

You need to electrically connect the base with +5V to activate the transistor.

You should include a low value resistor (100R) in seriies with the the base in case it becomes shorted to 5V which will destroy the transistor.

What resistance is the relay coil?

The base current might not be high enough to allow enough collector current to flow to activate the relay, in which case you could add another transistor to increase the gain.

 

[audioguru]

Static electricity will destroy the transistor.
Without having a flyback diode across the coil of the relay then the high voltage produced by its inductance when it turns off will also destroy the transistor.

All you are doing is pulsing the relay at the mains frequency (your body picks up mains hum). The relay operates from smooth DC, not the pulsing DC it is getting.

 

[Hero999]

Static electricity will destroy the transistor.
Without having a flyback diode across the coil of the relay then the high voltage produced by its inductance when it turns off will also destroy the transistor.

I can't believe I missed that.

Damage from static can be mitigated by adding a diode in reverse parallel with the base and a series resistor which should hopefully reduce the current of any static discharge to a safer level.

The transistor could also be replaced with a MOSFET which has the advantage of not requiring any current to activate the gate.

 

[seefeldt81]

I can't believe I missed that.

Damage from static can be mitigated by adding a diode in reverse parallel with the base and a series resistor which should hopefully reduce the current of any static discharge to a safer level.

The transistor could also be replaced with a MOSFET which has the advantage of not requiring any current to activate the gate.

I replaced it with a MOSFET but when I rest my hand on the touch plate instead of being constantly on it rapidly turns off and on. Any ideas on how to get a constant on?

**broken link removed**

 

[audioguru]

The absolute max allowed voltage for the gate of your Mosfet to its source is only plus or minus 20V.
Static electicity from your finger can easily exceed 2000V which will instantly destroy the Mosfet.

In addition to static electricity your body picks up Mains hum at 50Hz or 60Hz. It will turn the Mosfet and relay on and off at 50Hz or 60Hz.

 

[joemcder]

I replaced it with a MOSFET but when I rest my hand on the touch plate instead of being constantly on it rapidly turns off and on. Any ideas on how to get a constant on?

**broken link removed**

MOSFET have a relatively high turn on voltage and are not designed to connect directly to a weak signal. You may need to add another bipolar FET to drive the MOSFET.

 

[joemcder]

Another point I just noticed. You seem to be drawing power from USB. Do NOT connect this circuit directly to your laptop or other computer! If you must use USB be sure to put a POWERED USB hub in between.

 

[olly_k]

How about this...