pf trimmer

[sillypig]

hi all im making a transmitter circuit it requires a 10 to 40pf trimmer but i don't have time so i was just wondering if i could replace it with a 20pf capacitor instead of the trimmer would this work ?
thanks

 

[Sceadwian]

Sure, but you lose the ability to tune it, you'll be stuck with whatever frequency it decides to make.

 

[sillypig]

yes i understand that but im going to use a scanner to locate the signal i just want it as a signal to my scanner if some one breaks into my shed again like a wireless alarm

 

[Reloadron]

yes i understand that but im going to use a scanner to locate the signal i just want it as a signal to my scanner if some one breaks into my shed again like a wireless alarm

Circuits like that are not exactly stable. First it looks to be a basic FM transmitter (88 to 108 MHz) so if you place the cap you have in there it could easily come up on top of a local FM radio station that could overpower it. Then you could try messing with the coil spacing a little to change the frequency. Next problem is stability. Depending on temperature and humidity the transmit frequency you have today may not be the same tomorrow. Also, I never built the circuit so have no clue how well it works and if it works the effective range.

The guy in these forums good with tiny low power FM transmitters is AudioGuru so he may come along with some thoughts and suggestions.

Ron

 

[ericgibbs]

hi all im making a transmitter circuit it requires a 10 to 40pf trimmer but i don't have time so i was just wondering if i could replace it with a 20pf capacitor instead of the trimmer would this work ?
thanks

hi,
One way that is often used for 'low' value cap trimmers, is to use two short pieces of solid conductor insulated equipment wire.

Say each about 1inch long, solder one piece of wire to each end the fixed cap, twist half of the wire together.

By changing the fixed cap and the number of twists on the wire it is possible to tune a circuit.

The best way is to buy a 10 to 40pF trimmer... Whats your location, if its in the UK, I could post you a couple of trimmers, FOC

 

[audioguru]

The circuit you found uses the first three transistors to turn the transmitter on and off over and over which is not what an alarm should do.
Its frequency changes as the supply voltage from a battery runs down.
Its frequency also chages when something or a burglar moves toward or moves away from its antenna since it is connected to the tuned LC circuit.
It might not go far enough because it is low power and its antenna is too short.