Saw Tooth Supply for FM Radio Jammer - Page 1
Forum section: electronics / projects

author: Potato
author_rank: Contributor
timestamp: January 02, 2014, 02:22:07 pm
content: First post, although I've been lurking around for the past 18 months. I have caught the electronics bug, and have gradually built up a small hobbyist lab including a Fluke 17B, Rigol DS1102E and a cheap Tenma power supply. I've been gradually teaching myself stuff, and learning how to use particular components. As part of this process I have been constructing an FM Jammer, and have now encountered a difficulty in designing an improvement to the project.The jammer uses a 555 to generate a ~kHz square wave which modulates the base of a 2N2222 transistor. The collector of the transistor is connected to an LC oscillator tuned to MHz FM radio frequencies. A similar FM jammer can be found athttp://www.circuitdiagram.org/555-tracking-transmitter.html. FOr my version I have left out the trim capacitor and instead vary the supply voltage to tune the circuit.The circuit works very well, and my use of the supply voltage to tune the circuit gave me an idea. If I supplied the cicuit with a saw tooth voltage (5V to 10V ramp at ~1Hz) then the circuit would scan across the radio spectrum, creating a broad spectrum jammer and saving the need to tune it.So, I thought I would use a 555 again to generate the supply voltage as a saw tooth. Using a transitor based current source to charge a large (1500uF) capacitor, with the 555 set to discharge the capaitor through pin 7 when the capicitor raches 2/3 of the supply voltage, and then allow recharging from 1/3 of supply voltage. This was based on a circuit fromhttp://www.electronic-circuits-diagrams.com/oscillatorsimages/oscillatorsckt2.shtml, but with a bigger capacitor (and corresponding resistor). This circuit worked fine on its own, generating a nice saw tooth from 5V to 10V when I connected a 15V supply. I now hooked it up to my FM jammer as the power supply to the FM jammer.I had hoped that by using a larger capcitor on the 555 saw tooth supply, it would be able to supply the FM jammer with the required current. However, it just doesn't work: as soon as I connect the 555 saw tooth to the FM jammer, the saw tooth disappears. I assume there is a limit to how big a capcitor it is wise to use since it much discharge through the 555, so I'm not going any bigger at the moment.Can anyone suggest where I am going wrong, or indeed a completely different starting idea for supplying a saw tooth. I'm really after a simple solution I can build and understand (even if it doesn't work too well) rather than a perfect solution that teaches less.Cheers

author: GeoffS
author_rank: Unknown
timestamp: January 02, 2014, 02:39:57 pm
content: I don't image such a device is legal anywhere.What's its purpose?

author: Potato
author_rank: Contributor
timestamp: January 02, 2014, 02:43:34 pm
content: It only has a very limited range (ie a few metres), just like the devices you can get to boadcast your MP3 player to your in-car stereo; therefore, legality isn't a problem in my duristiction. Its purpose is purely as a fun learning exercise, I'm building it on a breadboard and once it is working I'll probably take it appart again (or more likely, add to it / improve it).

author: Rerouter
author_rank: Super Contributor
timestamp: January 02, 2014, 02:43:54 pm
content: buffer your sawtooth with an op amp that can power your jamming circuit,

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 02, 2014, 02:45:03 pm
content: Google varicap.

author: Potato
author_rank: Contributor
timestamp: January 02, 2014, 02:46:41 pm
content: Quote from: Rerouter on January 02, 2014, 02:43:54 pmThanks.Just to check my understanding. You mean using an opamp where the output is connected back to the non inverting input directly?

author: eripaha
author_rank: Contributor
timestamp: January 02, 2014, 02:46:52 pm
content: http://www.afrotechmods.com/cheap/jammer/jammer.htm

author: SeanB
author_rank: Super Contributor
timestamp: January 02, 2014, 02:48:25 pm
content: My thoughts exactly. As to the sawtooth I would recommend a darlington connected transistor to buffer the sawtooth so that it will be able to supply more current without loading the generator. A pair of 2N2222's will work here for up to around 100mA.Not recommending using a radio jammer unless you want to annoy the gents in blue who will come and take you away and confiscate all your gear. Almost every country has a rather dim view on deliberate radiators interfering with legitimate fee paying users of the radio spectrum. The fines can be very high even for inadvertant radiators, deliberate is classed like pirate radio and leads to a cell.

author: dannyf
author_rank: Super Contributor
timestamp: January 02, 2014, 02:52:07 pm
content: There are tons of circuit to generate a sawtooth out of a 555, basically following your approach. The issue is with integrating it with your transmitter: you will need to apply the sawtooth on a varicap in lieu or in conjunction with TR in your circuit, in order to modulate the transmitting frequencies.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 02, 2014, 03:34:19 pm
content: Potato,Quote from: Potato on January 02, 2014, 02:22:07 pmIf you arrange the oscillator to sweep the entire FM radio band at 1Hz rate, you will not jam anything because you will sweep through the signal that you are trying to Jam.Jay_Diddy_B

author: edavid
author_rank: Super Contributor
timestamp: January 02, 2014, 05:01:05 pm
content: You don't need an op amp or a varicap.As SeanB says, you do need to buffer the voltage on the capacitor with an emitter follower - emitter provides the transmitter's supply voltage. I doubt you need a Darlington though.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 02, 2014, 06:23:32 pm
content: Quote from: edavid on January 02, 2014, 05:01:05 pmSure, don't use a varicap, a 100K resistor and a 1nF capacitor to control the frequency in a normal way but go build a buffer to sweep the supply voltage...