A Photo Transistor Circuit Issue.

[electro_geek]

I am having problems getting this circuit to work. I am using a infrared LED as a signal for a photo transistor to create sound that can be heard through a speaker. The infrared LED is contacted to an PNP but it is not getting the voltage from the collector of the PNP to create an signal. What can I do? I have attached the schematic for the circuit.

 

[betwixt]

In that schamatic the IC output, when high, must be very close to 9V. what type of IC are you using and are you running it from a 9V supply line?

It would be more usual to use an NPN transistor with it's emiiter to ground and the LEDs in the collector.

Brian.

 

[kak111]

And add a series resistor 1k...2.2k from IC output to transistor base.
This limits current from base to IC output to 8...4mA.

 

[electro_geek]

I am using a 4011 Quad NAND Gate for this circuit.

 

[betwixt]

I don't think the base currentis the isue in this case although a resistor is certainly advisable. In order for the transistor to turn off, the output of the 4011 must be higher than 8.4V. If it is running with a supply voltage less than 9V it will not be able to tun the transistor off and it will conduct all the time.

A better way of driving the LEDs is to connect the 4011 output through a resistor as kak111 says but into the base of a NPN transistor. connect it's emitter to 0V (supply negative side) and fit the LEDs as they are but between the collector and 9V line. It will invert the signal but it removes the need for the supplies to be identical.

Brian.

 

[Raza]

Hi,
I can see your LEDs being as IR but not sure about the Photo TRANSISTOR is also an IR sensing, white light sensing or what ?
Be sure abut the Photo transistor is an IR SENSING, else it may not work.