[Solved] Detecting wire presence with a transistor


I have a problem with my project that I just can't figure out. I have made the project and it works as initially intended perfectly - it's a remote firework igniter that when an RF signal is received, it activates a transistor which connects ground to +5 power source to fire off an e-match.

The problem that I have is that sometimes the e-match isn't put in properly and I have no way of knowing until I actually fire the thing, so as an additional feature I'd like to have a way to check if the e-match is put in properly without firing it. AFAIK the e-match requires ~100mA to ignite (maybe a bit less).
How can I modify my circuit to do that? Some suggest reading analog input....

Attached is my schematic and PCB layout of the receiver unit.

And if anyone's interested, I have a whole article on it on my blog: Remote detonation device for airsoft - REMDE - Kulverstukas's blog

Is the e match connected where is says 'fuse connector'?

I suggest a 10k resistor across the TIP122 so a tiny current will flow through the match, not enough to fire it. If you then use a digital input connected to the TIP122 collector it should be high with the match connected and low if it is not.

It might be best to use a 10k resistor as a precaution between the collector of the TIP122 and the input.

Add a connection from Fuse Connector pin 1 to an ADC input, see if it has 5V there.
Why did you not use any 0.1uF caps of VCC and AVCC?
You lose a lot of voltage with TIP122 as the fuse current sink. I would have used a Low Rds N-channel MOSFET instead.
AOI4286 for example

Your board doesn't seem to have '328P pin 14 connected to NRF24L01 pin 8.

Pin 8 is not needed for my purposes, so I left it disconnected :slight_smile:
I'm not very good with electronics to know the small differences between these components, and haven't heard about using caps on those lines either. Can you explain the benefit?
About the connector: that makes sense. Do I need to connect it through a resistor and simply do analogRead on where I connect it?

@PerryBebbington: I didn't quite understand what you mean. Can you draw a little what connects to where? :slight_smile:

Like this (I hope that makes sense).

Thank you @PerryBebbington but I tried CrossRoad's solution which is simpler and it worked really well. I didn't think of this myself :smiley:

@CrossRoads: Yesterday I search and read everywhere about the AREF and AVCC and I think I understand now what you meant, and to answer your question - well I didn't think it's needed because my circuit is pretty simple.
I also tried today to replace TIP122 (which is kinda an overkill tbh now that I realize) with an S8050, it worked ok so I'll use that for future units.

Thank you all.

Thank you all.

You’re welcome, glad you got it working.

++Karma; // For letting us know how you got on.

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