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Unwanted pulses

G

Gary

Jan 1, 1970
0
I am trying to steer the output of an infrared controller to one of eight
devices. For the steering circuit I've used a 74htc138. Each output is
connected to a pnp transistor (mpsa56) through a 5k resistor. The
transistor is used to supply 5V to the ir emitter. All 8 emitters are tied
together and into an npn transistor (pn2222a) that generates the pulses.

One of the outputs of the 138 is always selected providing 5v to the
appropriate emitter. Measuring the connection between the mpsa56 and ir
emitter I get 5v for the selected emitter and zero for the other 7. However
on a scope, I can also see the ir pulses at this connection on all of them.
I've used visible ir emitters and they do not light but the devices, on the
unselected emitters, can sometimes detect the pulses and react.

How can I isolate the other seven emitters?

Thanks
 
E

Ejalogic12

Jan 1, 1970
0
I suspect that your power source may have poor isolation. Try putting a larger
value capacitor from ground to V+
 
U

Uns Lider

Jan 1, 1970
0
I am trying to steer the output of an infrared controller to one of eight
devices. For the steering circuit I've used a 74htc138. Each output is
connected to a pnp transistor (mpsa56) through a 5k resistor. The
transistor is used to supply 5V to the ir emitter. All 8 emitters are tied
together and into an npn transistor (pn2222a) that generates the pulses.

It might help to change the circuit topology a little. What's the frequency
of your signal and how much current does each IR-emitter use?
I've used visible ir emitters and they do not light but the devices, on the
unselected emitters, can sometimes detect the pulses and react.

Another thing to consider is the light may be leaking from one IR-emitter
to places where you don't want it...

-- uns
 
L

Luhan Monat

Jan 1, 1970
0
Uns Lider wrote:

Another thing to consider is the light may be leaking from one IR-emitter
to places where you don't want it...

-- uns

Yea, watch that IR. I found it going right through a piece of black
plastic electricians tape.
 
U

Uns Lider

Jan 1, 1970
0
Yea, watch that IR. I found it going right through a piece of black
plastic electricians tape.

Wow! I found that interesting so I just did a little experiment. I put a
square of black vinyl electrical tape over the IR LED on a remote control
and pointed it at a video camera. The IR emissions were clearly visible
even through the tape. Then I proceeded to turn on my CD player with the
remote control -- electrical tape still in place! After I added a second
layer of tape, the IR was no longer visible through the video camera, but
I was still able to turn the CD player on if I held the remote within
about an inch of the player's sensor.

I then repeated the test with some special black tape used for
light-proofing in darkrooms. With one layer of that tape, the IR was
still just barely visible through the video camera, and the remote could
turn on the CD player at a distance of several inches. Two layers of that
tape prevented the remote from working at any distance.

-- uns
 
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