Help with a transistor inverter circuit

[rebeltaz]

I have been in consumer electronic repair for 25+ years, so this is embarrassing, but...

I am working on a circuit where I need to trigger a 555 when a beam is broken. To get the range I need, I am using a darlington NPN amplifier and I have come up with this portion of the schematic (the rest of the circuit is working fine), which almost works perfectly. I plan on tying the 555 input to the junction of the LED/capacitor/resistor/collector. The problem is that in this configuration, the signal goes HIGH when the beam is broken. I need the signal to go LOW when the beam is broken.



In my other circuits, I have used the NPN to switch a PNP to provide the inverted signal. For some reason, no matter what circuit I come up with, nothing works as expected.

Also, I know the formulas for calculating the base resistor to a transistor configured as a switch, but they all rely on knowing the current the transistor will be switching. If I left the LED out and only switched the 555 input, the current would be infinitesimal. How would I calculate the resistor in that case?

I hope you guys can help me as I am running low on hair to pull out! Thanks.

 

[(*steve*)]

Try something like this

 

[rebeltaz]

After I posted this, I thought about adding another transistor after the between the NPN in my circuit and the 555, similar to the circuit you provided. I think that will work. I'll try it tomorrow. Thank you!

 

[Colin Mitchell]

Here is the circuit:

 

[rebeltaz]

Thanks guys. I actually need more amplification than that. Here is what I am using - works great now