HP Clock

Friday, March 23, 2012

More DIY IR Jammer

I did breadboard and test the DIY IR jammer last weekend but it’s taken me this long to carve out a minute to post the results.  The schematic is correct and only took a few minutes to re-create on the plugboard.  For power I used a small bench supply set to 6 volts to duplicate four AA cells.  When I powered up the circuit it oscillated just as predicted, but the frequency of oscillation did not agree with LT Spice.  The first time I entered in the schematic I just picked an arbitrary Op Amp from those provided by Linear Technology.  That is what caused the discrepancy.  I tracked down a TL082 model and that provided results that are nearly identical to the actual circuit.  Here is a screen shot of the LT Spice schematic including the spice directives used to add the external model.

So the lesson here is use the correct model for accurate results.  I’m including the TL082 model I used in the body of the post.  Just copy and paste it into a text file adding a .sub extension if you want to do your own simulation.

TL082 Spice Model

* (REV N/A)      SUPPLY VOLTAGE: +/-15V
*                | INVERTING INPUT
*                | | POSITIVE POWER SUPPLY
*                | | | NEGATIVE POWER SUPPLY
*                | | | | OUTPUT
*                | | | | |
.SUBCKT TL082    1 2 3 4 5
  C1   11 12 3.498E-12
  C2    6  7 15.00E-12
  DC    5 53 DX
  DE   54  5 DX
  DLP  90 91 DX
  DLN  92 90 DX
  DP    4  3 DX
  EGND 99  0 POLY(2) (3,0) (4,0) 0 .5 .5
  FB    7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6
  GA    6  0 11 12 282.8E-6
  GCM   0  6 10 99 8.942E-9
  ISS   3 10 DC 195.0E-6
  HLIM 90  0 VLIM 1K
  J1   11  2 10 JX
  J2   12  1 10 JX
  R2    6  9 100.0E3
  RD1   4 11 3.536E3
  RD2   4 12 3.536E3
  RO1   8  5 150
  RO2   7 99 150
  RP    3  4 2.143E3
  RSS  10 99 1.026E6
  VB    9  0 DC 0
  VC    3 53 DC 2.200
  VE   54  4 DC 2.200
  VLIM  7  8 DC 0
  VLP  91  0 DC 25
  VLN   0 92 DC 25
.MODEL DX D(IS=800.0E-18)
.MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1)

Here is the revised spice plot followed by the actual waveforms that I recorded with my oscilloscope. 

It’s easy to see how well the simulation agrees with the hardware.

I came up with two simple tests to confirm that the circuit was actually emitting infrared pulses.  The first test was to simply point a video camera at the LEDs while the circuit was on.  This picture shows this basic setup.

Even the cheapest of video cameras will show infrared light.  The infrared light shows as a pale blue in this picture I took of the video monitor.

The next test was equally as easy and simply involved attaching a photodiode to an oscilloscope, and then pointing that photodiode at the IR LEDs.  The resultant pulses can be easily seen here.

Did it work??

I took the lab supply with the plugboard on top into the living room and set it on the coffee table and flipped the power supply switch on.  Admittedly my setup is not very covert, but it did successfully disable the remote's ability to change channels.

Thursday, March 15, 2012

DIY IR Jammer Missing Information

The GREAT CREATE at Radio Shack is truly a well-intentioned and worthwhile exercise for getting kids interested in electronics and technology.  It harkens back to the Radio Shack I new as a child where I could find all the bits and pieces for my projects.  I found a brochure in the local Radio Shack that supposedly provided instructions on building a infrared remote jammer. 

I say supposedly because on closer inspection the brochure was filled with errors and missing information such that any kid trying to build it would have been met with disappointment and frustration.  Projects like this done properly can be an inspiration that jumpstarts a career in science or engineering. Improperly presented they can turn a young mind away from a potentially great learning experience.  With that in mind I posted the missing schematic and an explanation of how the circuit works, along with a spice simulation.

The Schematic;
The schematic breaks down into two major blocks with the first being a relaxation oscillator built around the TLO82 FET opamp, and the second being a current driver that uses the BJT transistor to convert the output of the oscillator into current drive for the infrared light emitting diodes.

The relaxation oscillator works by using the opamp as a comparator.  The charging and discharging of the RC network comprised of R11, R10, and C3 which is fed into the inverting input of the opamp causing the output to change states positive to negative every time the node of R10 and C3 reaches the 3 volt compare point set on the non inverting input.  That 3 volt point is is set by the voltage divider R12 and R5.

The diode driver works by AC coupling the signal out of the opamp through C1 directly into the base of the transistor.  The resulting pulses drive the transistor from cutoff to near saturation.  10 ohm resistor R7 provides current limiting through the diodes.  

Spice Simulation; 
All of this can be demonstrated by viewing the graph I created using LT Spice.  LT Spice is one of the best free programs available for simulating electronic circuits and can be found on Linear Technologies Web site.  I highly recommend it.  As for the simulation it shows the first 200 microseconds of the circuit, at which point it reaches steady state oscillation.  This graph shows the RC circuit in red, the opamp output in blue, and the diode current in green.  I may take time this weekend to build up the circuit to see if it works and matches the simulation.  If I do there will be pictures.

Critique of the Brochure;
  1. The two pictures of the circuit are rotated 90 degrees making it difficult to trace out.
  2. The shopping list is wrong and calls out only two 10k resistors, while the circuit requires 6 10 K resistors.
  3. No schematic or no link to a schematic.
  4. No explanation on how it works.