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I am helping someone out over at aviation stack exchange -- they want a signal led for their gps receiver to do a preflight check to make sure it is working. They basically want to be able to press a button and see if the tx line is hot (as it should be) with a signal led. I suggested they use a transistor so as to not draw too much current from the tx line. Here is a link to the original post:

https://aviation.stackexchange.com/questions/42812/how-can-i-check-that-the-gps-receiver-is-sending-data-to-the-transponder/42826#42826

Here is a link to the datasheet of the PNP transistor i use: https://www.centralsemi.com/get_document.php?cmp=1&mergetype=pd&mergepath=pd&pdf_id=2N4402.PDF

And here is what I suggested. Is there anything in this circuit that would damage their equipment (or any mistakes I made in the circuit)?


Your GPS puck (GlobalSat BR355) runs on 5v and the UART line idles high (it is pulled high when there is no traffic). The first thing you need to do is find the power lines that go to the gps puck -- connect wires to these lines (5v and gnd) and run them to the spot where you want the button/led. Then make this circuit:

enter image description here

The value for R1 should be 120 ohms (which will yield a little under 20ma of current through the LED, assuming your led has a voltage drop of 2.1v (which the ones in the link I provided do have))

Just wire these components up according to the picture and solder them together -- I'll leave how to mount the switch and the LED up to you but make sure once you are done you cover all the exposed wire with glue and/or electrical tape and/or liquid electrical tape so they don't accidentally short out. Also make sure no exposed wires within the circuit are touching where they shouldn't touch (especially make sure you separate the pins of the transistor because if they accidentally touch the circuit will not work and you may damage your gps receiver -- long story short be careful in your wiring and make sure nothing is touching that shouldn't.

P.S. if you want you can move the button between 5v and LED1 if that makes it easier for you to mount; that would look something like this:

enter image description here

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    \$\begingroup\$ Well that's a long question. \$\endgroup\$ Aug 9, 2017 at 20:37
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    \$\begingroup\$ And the schematics are huge (yet the circuit is small) \$\endgroup\$
    – Eugene Sh.
    Aug 9, 2017 at 20:38
  • \$\begingroup\$ yeah -- ignore that... just look at the schematics. It should be a pretty simple circuit but I want to make sure I've got it right. In the rest of the text I'm just giving links to my recommended parts. \$\endgroup\$ Aug 9, 2017 at 20:38
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    \$\begingroup\$ If you think some parts of your question are irrelevant and can be ignored - remove them. \$\endgroup\$
    – Eugene Sh.
    Aug 9, 2017 at 20:40
  • \$\begingroup\$ I shortened it. \$\endgroup\$ Aug 9, 2017 at 21:07

2 Answers 2

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First schematic stinks.

With the button NOT pressed, when the TX line goes low, the base-emitter junction acts as a forward biassed diode, and draws enough current to light the LED from the TX pin. This is probably not what you intended.

I would use the button to connect TX to the circuit, so that there is normally no connection to the GPS signals, and perhaps use a very high value resistor (1M) from base to +5V to hold the transistor off when the button is not pressed. Other details are reasonable, Olin's comments are good (and I see he's revised his answer to cover the switch location)

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  • \$\begingroup\$ Thanks -- if I forgo the pullup resistor on the button (just to keep it simple -- this circuit is for a pilot not an electrical engineer and I'm not sure how comfortable he is with soldering) will the floating base cause the led to light erratically at all? I suspect it's fine without it as transistors are powered by current not voltage (like mosfets) and I doubt any electrical activity in the wire would be able to supply enough current to close the E-C junction. \$\endgroup\$ Aug 9, 2017 at 21:21
  • \$\begingroup\$ It probably won't light up in the absence of electromagnetic interference ... a nearby VHF transmitter for example. And if that's a problem, you may need something a bit more robust than a 1M resistor (bypass capacitors, ferrite beads) He'll need to do a radio check to test that. \$\endgroup\$ Aug 9, 2017 at 22:23
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There was way too much to read, so I'm only responding to the first schematic.

You have roughly the right concept, but some details are wrong.

Let's check the resistor value. The LED drops 2.1 V, and figure about 700 mV E-B drop in the transistor. Starting from 5 V, that leaves 2.2 V across the resistor. For 20 mA, the resistor would be (2.2 V)/(20 mA) = 110 Ω. Your 120 Ω is therefore very close to the limit. I'd use 150 Ω to provide some margin. You'll barely notice the loss of brightness in the LED.

The outright wrong part is how the switch is incorporated. When the collector of the transistor is floating, the transistor acts like a diode E-B. The signal will still try to light the LED, but there won't be any gain from the transistor to help it. Put the switch in-line with the base, not the collector.

A more fancy circuit would detect toggling activity, but what you show is one step better than nothing.

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