Let's Make Robots!

do you think this would work if i wired it up to a picaxe 14M

ok this is a circuit i have come up with to try and not use a motor driver for my picaxe,

i will have the base transistors connected to pins of a picaxe 

i will appreciate your input 

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If you are willing/can order parts, is there any reason you would be apposed to simply using a motor driver chip? L293D for example, is sorta the "standard" and would run that motor just fine.

Oh, and again, closer... You show in your schematic using 8 pins to control the hbridge and ask about 2 transistors on one pin. --Yes, this is the way it is done. The connections would be "diagonal", one pin controls 2 transistors. --Check out the schematic that Dannyv posted above.

I can't read your writing and a few things are missing. Skip it for now, doesn't matter.

Ok, here we go:

I assume you did some looking around and found a schematic for a simple h-bridge. From there, it seems you have copied part of it and mixed it around a bit. Understand, I am not trying to make fun of you here, but for the life of me, I can't figure on how you got to where you are now. If I sorta knew the steps you took to get where you are now, I might be able to steer you a bit better.

I guess I will start you with this:

This is what it is supposed to look like. Note the resistors on the bases of the transistors and the diodes. Also note that this is for one motor and allows forward and reverse. You will need (2) of these set-ups for (2) motors but neither one will share any pins with the other (except for power and ground).

From here you just need to ask questions. "Is this right?" is not going to work much more --the more specific the question, the more specific the answers.

For a few days i am wondering how to do this. I don't understand why in practice this resistance is so important ? Stupid me !

They say it is pulling up my current, but which one - those of the parallel port's pin i am using or thou of transistor ?

Isn't the current on Gate triggering the flow ?

"Lets say" i have a motor rated at 30V, consuming 4 Amps of DC .

So the next thing to ask is - 4A is normal, but how much should i need to turn opossite with full load ?

- This answer i suppose should answer for the transistor's rating (- 20)% probably.

Next : as i understand  in the diagram that transistor's ratings have no matter(If they are below supply) and 1KOhm is good enough when using power supply in range between (2.2 to 9,6)V.But what about 30V, or 48V with 20A ?

And about Ω again 

Other place i see the formula R = 5 / {[(Vcc / Rc) / B] * 4 } , where B is the gain of Transistor, Rc should be at my motor, and 5 are the volts on Gate. 


So please, Somebody help ! I need to find a way to calculate it i want to make this motor spin.

1. I don't think you'll get a reply here, except for me, telling you to start another post.

2. Gates are for FETs. Bases are for BJTs.

3. There is a lot going on in your post. Try to single out a specific topic and use it as your post subject. It will get better views than the vague "Please help" title.

Let's get that motor spinning.


Thank you, Jack ! You are a nice man ! 

Tomorrow i will take advantage from your advice.



Right I have:


2n3904 NPN transistors about 100 of them

I don’t have any PNP transistors as of yet have you got any to recommend me?


For motors I was thinking of getting these






I haven’t used diodes much I would need a bit of help on them


And thank you for the H bridge circuit


I have used the one you showed me and used my wonderful skills on paint to draw this, I recon I have probably missed something obvious   







so i would be able to operate two transistors on one pin would that be acceptable? 


No. This will not work.

You are sorta close to making an H-bridge set-up. I would start there --searching for "H bridge" or "H bridge schematic".

will this work now ? 

That is an improvement. You are missing:

  • A resistor (about 1k is good) between each Picaxe output pin and the base of each transistor.
  • The diodes in the schematic CtC posted are their to protect your circuit from the back-EMF that will happen when your motors stop. You should definitely include them.

Other than that, this will work, with the following extra notes:

  • The transistors you use must be able to provide enough current for your motors. I don't think you have told us how much current your motors pull (free running and stall), which transistors you want to use.
  • Notice in CtC's diagram how two transistors are NPN and two are PNP? That's good practice because the H-bridge won't short out if you wind up with all output pins set high due to a microcontroller reset or coding error.

Search this site for 'h-bridge matrix' for some good examples.

That way the motors will get 9 volt positive but no negative signal. In your drawing you only used s1 and s3.


You have to connect the transisters crossed: In this example they are switches. If you close s1 and s4 at the same time, the motor wil turn. When s2 and s3 are closed the motor turns the other direction. NEVER turn switch s1 and s2 (or s3 and s4) on at the same time!! as it will be a shortcut and you will blow your transisters.