Can we use 2-20pF Trimmer instead of 5-18pF

[gaurav279]

Dear TOMI,

This is with reference to your reply posted on the subject of Aircraft Receiver in this forum.We have been working on the passive aircraft receiver circuit as mentioned by you.Although we gave been able to get almost all the parts,the Trimmer Capacitor of 5-18pF as mentioned in the cirucuit is very difficult to find in india... So Instead of this Can we use 2- 20pF Trimmer capacitor which is readily available in India...Also this trimmer has 3 legs ...so kindly guide us as to which of the 2 legs have to be shorted in order to fit it in the 2 pin area of this circuit.Also if we use 2- 20pF Trimmer do we require to make any changes in this circuit...

Kindly reply at the earliest as our project is on hold because of this...

Thanks & regards,

Gaurav
India

[Tomi Engdahl]

We have been working on the passive aircraft receiver circuit as mentioned by you.

You must be referring to this circuit
http://www.pcs-electronics.com/schematics/aircraft.pdf

.Although we gave been able to get almost all the parts,the Trimmer Capacitor of 5-18pF as mentioned in the cirucuit is very difficult to find in india... So Instead of this Can we use 2- 20pF Trimmer capacitor which is readily available in India...

I see no problem in using that different capacitor value.
Having a capacitor that can go somewhat higher and lower values than the original one means that the radio tuning range with that new capacitor is slightly larger than with the original design. Should not be problem here.

Also this trimmer has 3 legs ...so kindly guide us as to which of the 2 legs have to be shorted in order to fit it in the 2 pin area of this circuit.

Hard to say which one of the pins to connect when I don't have the data of the component you have or even a picture of it. Any two pins connected to circuit that would give that variable capacitance should work on circuit (with third pin not connected anywhere). If you have capacitance meter you can check which pins have the right capacitance value that changes as you want.

I have seen some capacitors where there has been two pins that are short circuited together. In this case either of those can be used as one end of that capacitor and the third pin is that other end of capacitor.

Also if we use 2- 20pF Trimmer do we require to make any changes in this circuit...

I don't think any changes would be needed.
If I were building that circuit myself and had the same situation I woudl just put the capacitor you have into that circuit.