ATtiny85 to control three 12V LED strips using TIP122

Hello everyone,

I am in the planning phase for a project that I need to complete in the next 4 weeks.

I am creating a military shadow box for my retirement from the USAF. Everything is powered from one 12V 5A power supply. A 5V voltage regulator is used to supply 5v power to the ATtiny85 and the PIR motion sensor. The output from the sensor is sent to PB2 on the ATtiny85, and this starts turning on the three 12V LED strips at different intervals using PWM output on PB0, PB1, and PB4.

I read somewhere that PB4 could be used as a PWM output if programed correctly.

I have programing experience in Arduino but this is my first ATtiny project.

Please let me know if my breadboard circuit is designed properly.


Aside from the lack of decoupling caps on the Tiny85 5V power pins it looks ok to me. Also double check the 5V regulator data sheet to make sure that there are not caps required on its input and/or output (ex. LM1117 regulators need 10uf on the output).

I would use low Rds logic level MOSFETS instead of Darlington bipolar transistors as they are more efficient. 180 ohm gate resistors and 10K pull downs on the gate to keep them off during reset.

Thanks for the help.

I am using the TIP122's since I have a bunch of them on-hand.

If I were to order MOSFET's in the future, what specific one would you recommend for this type of application?

What are decoupling capacitors? What size and how are they connected? What type of Cap?

Do I need to have a Pull-Down resistor (10K ohm) from PB2 to ground?

My 5v voltage regulator is the L7805CV

**EDIT In my first post I assumed that the 12V LEDs had built in current limiting. If that is not so, current limiting for the LEDs must be discussed. **

I can't recommend a MOSFET as you have not specified the LED current. The things to look for, in the data sheet, are that the continuous drain current is higher than your LED current, and the Rds is as low as possible. Also the gate voltage for the Rds spec is 5V or lower (logic level). I am sure that once the current is known, people will recommend their favorites.

For decoupling caps, please Google "Arduino decoupling caps". There is much written about that so I won't repeat.

Since bipolar transistors are current driven and the pin on Arduino are effectively open (hi-z inputs) at reset no current can flow to turn the transistor on. So, no pull down needed. MOSFETS are voltage driven and, voltage wise, a floating pin (no pull down hi-z) can be any voltage the pull down is required to make the pin LOW to keep the MOSFET off during reset. Then you set the pin to OUTPUT and it can't float anymore.

7805 regulators, according to the data sheet, don't require caps except under special circumstances (power supply a ways away, etc). It never hurts to add some decoupling (small ex. 0.1uf). Some storage (ex. 10-100uf) caps can smooth out bumps in the supply voltage from higher current spikes from, for example, LEDs turning on and off.

@sethmoening, do not hijack.

@sethmoening, do not cross-post.

Other thread removed.

Okay... Sorry about that. :confused: