Return To Our Home Page

WB1 - Crystal Controlled FM Transmitter
The Original Circuits Below Were Built and Sold -  In The Thousands To Law Enforcement. Their Original Research and Development Cost , By Today's Standards, Would Total - Well Over A Million Dollars.

 FM Crystal Controlled Transmitter Circuit

(C) 2012  radio-circuits .com
The WB1 Transmitter "WINDOW BUG"came about a few years back when a Sheriff down in Miss needed a device that he could stick on the outside of a mobile home on a back window out in the woods that would transmit all of the sounds on the inside of this trailer and then transmit those noises ( Including Conversations ) down into the woods to a remote location several hundred feet away. It works and uses a common vibration element. Can also be used on Doors and Walls. Middle to Upper VHF Band.

WB1 Window Transmitter
2011 By John S. Wilson Jr.

 Parts List: Circuit # WB1,  Window Bug - Xtal Controlled

 Resistors: 1/4 Watt

R1 = 1 MEG
R2 = 10K
R3 = 1 MEG
R4 = 10K
R5 = 47K
R6 = 470K
R7 = 220 Ohm
R8 = 10K
R9 = 470 Ohm
R10 = 33K
R11 = 4.7K
R12 = 100 Ohm
R13 = 10K

Capacitors: - Disc Or Electrolytic ( Electrolytic Shown with +/- )  All Low Voltage
C1 = 10 MFD
C2 = .001
C3 = .01
C4 = .01
C5 = .01
C6 = .01
C7 = .001
C8 = 4.7 MFD

C9 = .01
C10 = .01
C11 = 100 pf
C12 = .001
C13 = 4 to 20 Pf Trimmer ( Adjustable ) Small Surface Mount Type
C14 = 10 pf
C15 = 18 pf
C16 = .001
C17 = 4 to 20 Pf Trimmer ( Adjustable ) Small Surface Mount Type
C18 = 4 to 20 Pf Trimmer ( Adjustable ) Small Surface Mount Type
C19 = 10 pf
C20 = 47 pf
C21 = 4 to 20 Pf Trimmer ( Adjustable ) Small Surface Mount Type
CX = .001

Transistors: All From Digi-key Electronics

L2 and L3:
7 Tight Turns of # 26 Magnet Wire Wound tight on a Digi-key Electronics 3368K-ND Fiber Washer. The Inductance Of This Coil Is .056 uH

Antenna: 18 1/2 Inches of small flexible stranded wire

I usually build this circuit to operate between 150 to 165 Mhz. The Crystals I use Are The Digi-Key Electronics Barrel Crystals CA-301 Type which cost a couple of bucks per crystal. This circuit uses a X5 Multiplier and then a X2 Multiplier In the Final.

Example: If I use a 15 Mhz Barrel Crystal, the frequency at Q4s output is: Xtal X 5 Or: 75 Mhz into Q5. Q5 Multiplies 75 Mhz X 2 for an end frequency out of the final Q6 ( Desired Output Frequency ) of 150.00 Mhz.

Virtually any Fundamental Crystal between 14 and 17 Mhz will output a frequency of between 140 to 170 Mhz, if the crystal has a load capacitance of around 18 pf. If you wish to build this transmitter on A custom frequency simply use the X5 X2 formula below and order a custom cut crystal specifying an 18 pf CL.

Example: My Customer Desires this transmitter to function at an output frequency of: 158.250 Mhz?
( 1 ) Divide 158.250 times 2. This Equals 79.125
( 2 ) Divide 79.125 times 5. This Equals 15.825
I would order the crystal at a Fundamental frequency of 15.825 Mhz, tolerance of .001% and a Crystal Load Capacitance of 18 pf. When I tune my transmitter, my output transmit frequency would be: 158.250 Mhz.


Q1, Q2, Q3 and Q4 - All 2N3904 - NPN - Note, I use All SMD Types In This Circuit
Q5, Q6 and Q7 - All BFS17 -  NPN - Note, I use All SMD Types In This Circuit

This Resistor is a 470K. It controls the deviation or modulation of the crystal at VR. If the reception appears distorted on your receiver, increase its value. If more audio is needed, decrease it to around 100K. A trimmer resistor of between 100K to 500K could be used in place of the fixed 470K for an exact alignment of the transmitters deviation

NTE-613 Varactor Diode or a suitable replacement Or the Digi-Key Electronics MV2107

ON/OFF Switch:
Any, On / Off Switch Will Do - connecting the + side of the battery to the circuit

1.5 Volts D.C. The Negative Side Of The Battery Connects To The On/Off Switch to ground

Radio Shack # 273-073 Piezo Element. The Red Wire Connects To The + Side Of C1, The Black To Ground. Note: most any sensitive element Piezo Element will do.

21 Turns of # 30 Magnet Wire Wound tight on a Digi-key  Electronics3368K-ND Fiber Washer. The Inductance Of This Coil Is .150 uH