Tapping a circuit to sense when an SD card has been written?

Hi there, I come asking for help, perhaps because I'm being a nOOb, and perhaps because my question is a really hard search term.

I'd like to connect an arduino to an existing RX PCB inside an RC toy, and be able to sense when an SD card is written to. This will allow me to repurpose the RECORD button on the TX (which controls a feature I won't use) to do something else - see below:

I have a project where I'm modding a Nerf toy called a Terrascout (worth a google if you're interested). It's like a toy RC tank that fires nerf darts and has first person video with a screen on the transmitter.

The mod pathway for upgrading the Terrascout's flywheel powered blaster is well-trodden, it's all about intercepting the stock motor signals (via opto-isolation) and triggering a MOSFET to control beefier motors connected to a beefier battery - easy peasy stuff.

Where it gets interesting is that I figure I can put an arduino into the (well trodden) mix for finer a few features of my own. Perhaps voltage to the motors being tunable by PWM; controlling the dart pusher motor to fire 1, 3 or 5 round bursts each time the trigger (on the TX) is pulled, or tapped, or double tapped etc; or adding switches to sense various things.

What has me really excited, is that the Terrascout has one feature I won't use, but is controllable from the TX. It has a record button which starts recording from the onboard 480p camera (which streams live to a screen on the TX). The recording goes onto an onboard SD card.

I won't be using the above 480p recording feature because it's 2021 and I can strap a gopro to the thing. Buuuut, that leaves me with the tantilisign prospect of repurposing the REC button on the TX to do whatever I like.

I've read-up a little on the SD / SPI interface, and don't properly understand it. Apart from intercepting start bits, is there a simpler way to let an arduino know when the REC button on the TX is on or off (it latches)? I haven't yet seen the RX PCB in real life, but don't think there is anything easy like a red LED that lights up for recording.

Any ideas would be appreciated.

Nice toy. “tantilisign” poss for mods and hacks, yes.

My first thought was yours, a recording on LED you could hijack as a control signal.

Watching the SPI is prolly your best hope; but let us wait to see your good pictures of the top and bottom of taht RX PCB and any others in there.

Casual googling I will do only turned up not good enough pictures, but enough to make me wonder whether furious googling I won’t do might turn up an already been done brag or tutorial or knstructable or…

What a great time to be a kid. :expressionless:


Thanks A7, what a time to be a kid indeed.

You're right, teardowns of these are not common.

There are some pictures of the PCB in question on page 4 of the document here. But you may have already seen them.

PCBs seem to be a mix of epoxy blobs, surface mihnt and thru-hole components.

Given the lack of information out there in the Nerf community, and the number of these toys turning up at "yard sales" cheap, perhaps needing repair, I'm going to film a teardown of mine. Hopefully in the next week or so. I'll make sure I get good pictures too.

So I've got some pics of the RX board, and a short video

I've used a macro lens to try and capture chip details etc

I've not managed to identify anything useful, it seems others have had the same issue

blue green yellow orange

OK, nice pictures, in focus, but you managed to omit poss the most interesting legends, unless I missed it…

On one side there are wires blue, gree, yellow, orange, red and the silk screen legends begin RX...

No need to take more pictures, just post here what those are corresponding to the wires that obscure them in you photos.


Oh, I did indeed. My child woke up before I finished the teardown of the tank (TX unit is next for that) I didn't even see them under there.

So, for context, the RX is in the "blaster" (gun to anyone non Nerf-y) of the Terrascout, up front above where the barrel exits

That rainbow of wires goes down via a connector into the tank part, where the battery, traction motors and power switch live. Those have a separate PCB and various other componentry (like a cap accross batt leads)

The legend is quite hard to read, but looks like:

  • Blue wire - RX
  • Green wire - TXD
  • Yellow wire - 3.3v
  • Orange wire - Vout
  • Red wire - GND
  • Brown wire - Vbatt
  • Red wire on other side - "-out" ?
  • White wire on other side - Vbat
  • Pair of black wires on other side- B GND

Its a shame as they are aaaaaalmost legible at page 4 of the teardown linked in post 3 above

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