ATTiny85 + transitor servo control

To begin, I have only recently created my forum account, however I have been working with the Arduino systems for about a good two months. My knowledge with electrical systems is not the best, but I know enough to get by. I am using an Arduino Mega and also have been goofing with ATTiny85 microprocessors as of late.

I am working on a setup that operates two servo motors at random intervals. In my first build, the system runs fine on both the Arduino and the ATTiny85, in which the microprocessors do control the servos with flying colors. My concern was with the ATTiny85 build that the servos jitter due to a clocking issue (1Mhz vs 8Mhz the servo library was written for). The servo library claims to have compatibility at 1Mhz without jittering, but I'm leaving it out of the equation anyway to see if I can find a workaround.

The workaround would also, in theory, save on battery life. The idea is that ATTiny85 would send a signal (digitalWrite) to a transistor to control the power flow to the transistors on the control pin. While it worked with flying colors on the Arduino, the ATTiny85 can now only power one servo. Upon checking voltage, the 4 AA battery pack (6v) is going into the transistor and coming out as 4v, which is only just enough to control a single servo. Throwing another on simply causes undesired results (under-powered servos apparently ignore control signals and slowly move towards 0). Upon checking the output on the pin from the ATTiny85, I am seeing 5v.

As mentioned in my introduction, I am not from a major electrical field and am more of a computer programmer trying small work on microprocessor work. I'm hoping the issue isn't too hard to solve since the nature of the problem isn't too severe. I was wondering if anyone has an idea on what I am doing wrong and should look into to remedy the issue. If a schematic of the design is needed, I can write one up quickly.


i may be wrong here but without seeing a schematic, you can lose some voltage on the transistor if its not fully on (in witch case the voltage drop across the transistor will be greater than 0.2-0.3V which its usually the voltage drop its fully on aka saturated) - what resistor do you have connected between the base of the transistor and arduino's pin ?

Between the pins and transistors, there are no resistors. When working on the Arduino, it worked fine when using the 5v pin (I did have to give more power than what the USB was providing for the servos) on that, and on my prior build, which just gave power to the servos with nothing in between. I took a multimeter to the board and noticed the voltage was about 4v through the transistor with 5v going to the collector from the ATTiny85. Is it possible that it may be an amperage issue?


Yes, lets see the schematic and the transistor servo part numbers.

Something like that happened to me: couldn't drive my 2 motors with a L239D and an ATtiny84 in standalone. The measured tension on the attiny and L239D was less than 4V when supposed to be 5V. Issue solved with a couple of capacitors (100nF) between ground and Vcc pins.
Maybe it can help in your case too.