Recommend me a microcontroller dev board


Joined Apr 11, 2010
Without adding the ATTiny85, check the voltage from ground to just after the diode.

It should read 5.4VDV. if it more than that, you may need another diode in series. (Or add a 5V regulator circuit).

The ATTiny85 maximum voltage is 5.5V. Also, I recommended a 120Ω resistor. The resistor has to be rated for 1/4 Watt at a minimum. It appears you are using a 1/2W resistor, so you should be good there.


Joined Apr 11, 2010
If I end up running this from a straight +5v supply out of a USB wall-wart, what resistor value should I use for the LEDs in that case?
Anything from 68Ω to 100Ω.

The higher resistance runs the LEDs at 10mA. The lower runs the at 15mA. You could go as low as 51Ω, but I don’t recommend it. That is at the maximum rated current.


Joined Apr 11, 2010
There’s a lot of leeway in choosing a resistor for LEDs. That’s because a ‘typical’ LED will operate at anything from 5mA to its rated maximum, like 20mA.

You should operate an LED at less than it’s rated maximum current for a few reasons.

First, you extend the life of the LED.

Second, for many applications, you do not need the maximum brightness. Current determines the brightness. Running at 15mA is often enough and the brightness level is often not perceptible. For a display, running as low as 5mA is all that is needed.

Thirdly, an LED forward voltage can vary. Thus, if your LEDs have an actual voltage LEDs than the rating, the resistor may actually be higher than the calculated current. In some cases, this current will be too high for the LEDs, resulting in their failure. For this reason, you should use a resistor value which provides a lower than maximum rated current to the LEDs. Then if the LED forward voltage is low, you won’t blow up your circuit.

Some people use a potentiometer to test the LEDs and set the desired current. Then, they remove the pot from a breadboard and directly measure the resistance. With this value, they select a resistor from standard values.

Hope this additional information helps. I still recommend once you temporarily wire up an USB power supply, that you measure the voltage from the supply ground througn the resistor. It should be 4V.

Thread Starter

Jonathan Wilson

Joined Jun 9, 2019
Its all working now, all 4 strings are good.
Going to add a fuse and also look up how to use the ATTiny85 temperature sensor to prevent thermal overload.