I fly an ultralight aircraft. The GPS is a puck-type device from which the TX line runs to the mode-s transponder. The transponder does not readily show if it is receiving position data. The data is NMEA RMC sentences only, serial 4800 baud 1 message / second.

Is there any way to ascertain as part of the preflight check that the GPS is alive and sending some data (how correct is besides the point, for now)? I'm thinking along the lines of connecting an LED in parallel on the TX line which will flash to show data traffic if I press a test switch. Any ideas? Thx

  • 1
    $\begingroup$ You could buy a $25 usb TV receiver dongle and have it read the mode s data, to avoid having to modify your equipment. But then you must also always bring a laptop. $\endgroup$
    – Sanchises
    Commented Aug 9, 2017 at 8:35
  • $\begingroup$ An arduino might be a better choice than a laptop for that application. $\endgroup$
    – GdD
    Commented Aug 9, 2017 at 8:42
  • $\begingroup$ Silly of me not to include the data type and format. Added, thx. $\endgroup$
    – Srini
    Commented Aug 9, 2017 at 11:28
  • $\begingroup$ Even if the GPS is outputting NMEA sentences, that doesn't necessarily mean the transponder is receiving them. There could be a break in the connection at the transponder, and your device on the wire before that wouldn't catch that particular failure. $\endgroup$
    – Steve
    Commented Aug 9, 2017 at 12:37
  • $\begingroup$ What exact GPS puck/transponder do you have? Could you provide serial numbers? We need to know at what voltage the tx line is running on and whether it idles high or low (whether the voltage on the line is pulled high or low when there is no traffic). If you provide this I can help you out. $\endgroup$ Commented Aug 9, 2017 at 15:49

1 Answer 1


Your 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 150 ohms (which will yield 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))

Here is a link to a transistor that will work for Q1:


Here is a resistor that will work for R1:


And here is a link to a selection of LEDs you can use (all with voltage drop of 2.1v; Any of these should work; just choose a color and form factor that works for your application):


Here is a link to a selection of pushbuttons that should work -- just choose a form factor you like (look at the datasheet for the ones with 4 pins -- these are just two pairs of pins (each pair is connected) for mounting ease -- two pins can be harder to mount on a circuit board):


I personally recommend you use something like this for the button:


For this button here is a picture and a schematic (as you can see, the pins on either side of the flat side of the button are connected:

enter image description here

enter image description here

And something like this for the led:


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.

  • $\begingroup$ Thx Murey! The puck runs on 5V, so this would mean a higher R1? I will take precautions. I put details of the puck and transponder in an earlier comment. (I'm still learning to use the forum) $\endgroup$
    – Srini
    Commented Aug 9, 2017 at 19:54
  • $\begingroup$ By the way I made two changes -- one is that I realized the button must be inline with the base of the transistor for the circuit to work properly (so you can't move it around from where it is in the schematic now) and I changed the resistor value to 150 ohms just to put less stress on the LED (I updated the link to the resistor). $\endgroup$ Commented Aug 9, 2017 at 21:15

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