Arduino mini pro not able to upload code. Please help!

I have two Arduino mini pro 3.3v sparkfun boards. I have been using them for a while now, doing simple stuff like blink LED’s on and off and some work with small relays. The first one displayed this error: avrdude: stk500_getsync(): not in sync: resp=0x00 while trying to upload the same program I had been using before, but with a slightly longer delay in it. from what I have found so far, this doesn’t matter. Also, when I turn it on the green LED farthest from the reset button stays constantly lit, and the red LED closest to the reset button never lights I then switched to the second board, and after a few hours it had the same problem. I have tried many hours of Google-ing and troubleshooting with no luck:(

Thanks for the help!

Does either part feel warm/hot when powered up?

Was your last loaded sketch sending out lots of serial messages?

Nothing feels or felt hot.

This is my last sketch (the one when the second board stopped working)

int forwards = 10;
int backwards = 13;
void setup() {

pinMode(forwards, OUTPUT);
pinMode(backwards, OUTPUT);


void loop() {

digitalWrite(forwards, HIGH);
digitalWrite(forwards, LOW);
digitalWrite(backwards, HIGH);
digitalWrite(backwards, LOW);

I am using it to toggle a voltage reversing relay circuit that will automatically control a model railroad track.

I dont have the sketch I was using on the first one anymore, but It was using beatwrite in processing to control a model of a dance floor.

It seems a lot of people are having this problem. Is the mini pro just a faulty board?

Thank you so much for your time and help!

I don't think promini is faulty - I have 14 of them at my fencing club in scoring machines and a LiPo powered remote control, which spends most of its time in power down sleep mode.
All power up from same power strip, running lights, making sound, power them up 3-4 times a week for a couple hours at a shot in temps from 55F to over 95F.

Have any schematics we can review? The code looks pretty straightforward.

There are schematics attached. The first one if for the first circuit I was using, and the second one is for my current circuit. If it helps I can attach photos of the circuit, and what is happening when I try to upload it. Also, when I press the reset button, nothing happens. (the green light stays on and the red light stays off). The board is currently disconnected form the circuit to hopefully make it easier to trouble-shoot.

Thanks again!
-RedFox485 :slight_smile:

Running 2 relays in parallel off 1 IO pin? What’s the rating on the coils?
What’s the voltage at Vraw? You don’t really have enough capacitance there to supply the motor with current and keep a smooth input into the little 150mA regulator on the Promini.

I wasn't able to find any info on the relays, though I do know that the relays are very small and can only handle up to 15 volts. they don't seem to take much current at all and I was successfully able to switch two at a time with the arduino before it stopped working.
The relays say on them:

0490 0764

There are photos attached of the assembly and arduino. The first one is a photo of how the green light stays on and the red never turns on. the second is of the relay/ circuit assembly there is a added pot. to control motor speed. and the others are in my next post.

The Vraw voltage is 6v.

The cap. is just to even out any imperfections on the model railroad track like dust, or small bits of model railroad grass. And the rectifier is so that I don't fry anything if someone sets it on the track backwards.

Both the Arduinos were working just fine for about 20-50 tests, and now both have the same problem.

Thanks for the help!



Here is the model train it does in.

Thanks so much!

Here is the track that it runs on.

Also, I did check all of my connections before and after attaching the chip.

Woops! just caught this. In the schematic, the 100nf is actually a 470uf!



I think to hopefully avoid problems, I will switch to the atTiny85 and use an H-bridge.

Thanks for all the help!