Two schematic component questions..


Joined Nov 4, 2008
hey everyone, ive been looking at making this schematic, but cant seem to figure out what the value of L1 on pin 9 is.

Im not sure what the dotted lines indicate. Variable coil??

Also can i leave pin 19 open? or should it short to ground?

Thanks for your time!
The coil is a variable coil and is used to set the oscillator frequency. The varicap diode varies the frequency for FM modulation.
Pin 19 can be left open.


Joined Nov 30, 2010
K7 has it right.
A few details. Hopefully including at least one thing you didn't already know.
You could use a fuse on the power supply in case ZD1 ever becomes necessary. Theoretically, the 78L05 will protect the rest of the circuit, but no need to let it roast until you come back, possibly 3 days later.
I think BP means, "bipolar" but now you can get capacitors that big without using electrolytic types.
Both of the VR1's are audio taper pots.
When it comes to 3.3 pf, basically impossible to get anything except surface mount style.
The voltage regulator should be drawn like this to remind you that the capacitors need to be physically close to the regulator (to stop it from oscillating).


Last edited:


Joined Nov 30, 2010
Your attached circuit shows as :
Invalid Attachment specified. If you followed a valid link, please notify the administrator.
It works for me. Windows Vista SP2 with Firefox as a browser.

ps, thanks for double checking it, inwo.


Joined Aug 23, 2012
It works for me. Windows Vista SP2 with Firefox as a browser.

ps, thanks for double checking it, inwo.
If the 78L05 wish a 5V output then the input should be have 7.5V or some more, when the input only 5V, how many volts do you wish on the output?


Joined Nov 30, 2010
There is no intent to input 5 volts. The input is labeled, "9-16V DC". That indicates that the O.P. does not want the input to be only 5 volts. For 6 volts input (a 6 volt battery), the instructions for changing components and connections are on the upper left corner in the color red.

Thread Starter


Joined Jan 22, 2014
thanks for the responses guys!
how would i go about selecting the variable coil?
digikey offers many in different options, especially: Q @ Freq, and inductance..
Any ideas what values i should get?

The way i understand it is, this is what is going to adjust the range the 4 dips witches are going to control.

I realize its 0 to x, but i think x should be within "reason". That way the adjustment wont wont be so finiky right?
Last edited: