Electronic Circuits

Pen FM Transmitter Circuit - Crystal Controlled

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Pen FM Transmitter Circuit - Xtal Locked, 150 Mhz

This has to have been one of the most controversial FM Transmitter Circuits, I have ever produced. It did not start out as a pen transmitter and was originally built for a task force in Miami, Florida - as a "Baseball Cap" concealed NBFM crystal controlled voice transmitter.
It was originally built and surface mounted, in a Bayer Aspirin Box as shown below.

I engineered the original design as shown in the electronic diagram below,
based on a $1500.00 grant for the research and development - which back in those days was a lot of money. I went on to produce some 50 or 60 more of these units over the years and sold them at a price tag of $450.00 per unit.

In the early 90s a major Manufacturer that services the major intelligence agencies started producing a similar item, only at a price tag of $1,320.00  which they termed "A Baseball Cap Transmitter." Thru a friend in high places, I obtained a loaner and reverse engineered the circuit only to discover that they had reverse engineered my circuit? I was not bitter, since it is not uncommon in my industry, and a fact of life. It was just that they had major governmental contacts with millions in sales where I pounded the pavement from department to department in one state or another, city after city to survive.

The only difference that I noticed was that they had used a flatpack 6V battery that you will find in a Polaroid Camera film pack. I was impressed with their custom machined enclosure and the fact that they had pushed it's RF power output to 120 Mw with virtually no changes in the circuit itself.

Fast Forward  - How It Became A "Pen Transmitter".  I originally published this circuit back in the middle 90s on a website I owned called Narcbug.Com, which I no longer own. I received an email from a technician that headed up a drug task force down in Memphis, Tennessee. He had re-designed the circuit to fit in a pen. He sent me a working model and also photos and his layout that he had scaled on graph paper prior to building his board. I tested it, and it was fantastic to say the least!
Below is the backside of the board shown at it's actual size of 7 CM. Notice that only the Electret Element and The Crystal is shown on the backside. He did the SMD layout on the copper side and then drilled thru the board and mounted those two components on the backside.
Your Choice: Pen FM Transmitter or Layout the Circuit board in any configuration you wish. The Original and Complete Schematic is shown below. The electronic parts are shown on the electronic circuit itself.

baseball cap fm transmitter circuit
Modified From My Original "Aspirin Box" -  Baseball Cap Transmitter
Read Text Below
pen crystal locked transmitter circuit board
spy pen FM transmitter circuit
Crystal: Digi-Key Electronics CA-301   50 Mhz Barrel Crystal