Help needed for simple robot control circuit

The "magic word" you're looking for is "H-Bridge".

I didn't get any further than this point before hitting "reply", but with the context so far, I'm betting you're looking to have one or more DC motors spinning one way when the output of the OR gate is true, and "the other way" when the output is false?

Sure enough, that's EXACTLY what you're looking for - An H-Bridge (Google for it) will do what you're needing.

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Go get the gEDA package. The schematic capture app, gschem, works great.

The easiest solution is probably to drop in a dedicated H-bridge chip, unless you're really interested in doing a roll-yer-own effort.

has a good tutorial on discrete BJT H-bridges, if you're going that way.

For off the shelf, look at

or perhaps the SN754410 from TI

--
Rich Webb   Norfolk, VA

Thanks for the replies, but they haven't really resolved the problem. I hadn't heard of an H-bridge before, and I read through the tutorial from

that Rich Webb linked to, but if you check the schematic in the .gif, you'll see that I'd already designed a rudimentary H-bridge (it may not look terribly familiar since it controls 2 motors instead of 1, lacks protection diodes, has the motors external to the 4-transistor ring and doesn't have any resistors drawn in, but it seems to me to be the essence of what an H-bridge should be).

In the linked tutorial, the H-bridge is controlled by a Basic Stamp II microcontroller and, according to the tutorial; "This works because the processor pins become a connection to ground when they are outputting a logic 0. ". With my circuit though, a logic component as far as I know would never be able to supply a connection to ground as logic 0, logic 0 would merely be an absence of current, an open connection, not a path to ground. I'm fairly confident that I could get my custom H-bridge to work, with some modifications (the linked tutorial was very useful, and I was able to calculate that I'll need at least 4 of my little TO-92, 200mW transistors in parallel for each of the '4' transistors in the H-bridge to handle the 500mA or so that each motor will draw (200/0.7=286mA per small 200mW transistor), but I'd rather stick with BJT's for now, since I want to get a feel for the lowest level logic involved, there's something more satisfying about it to me and I have 200 of them to use up.

While I was typing this though, I think I figured out how to do what I wanted. Using 2 NPNs, 1 PNP and a couple of high-value resistors, I designed the switch I was after so that I could switch between +5Vcc and ground rather than just +5Vcc and 0V. It's 04:28 here, so my brain's kind of fried and I could just be imagining I've solved it by now, but if it still makes sense to me in the morning, I'll make another .gif of it and post it here. It looked pretty obvious once I'd done it, so hopefully that's a good sign.

Thanks to Rich Webb again for the link to gEDA, the electronics design package. Unfortunately I'm just learning to use linux at the moment, and haven't yet figured out how to compile packages from source (which I'd need to do since I run mandrake 10.0). I'll keep it in mind for when know how to install it.

There are specialized chips to deal with this problem. If you aren't making robosaurus, a TI SN754410 is probably what you want. For bigger projects, you can use discrete transistors; if you go this way, don't use BJTs, use MOSFETS, since the drive required for motors will be easier to deal with by logic gates.

If you look around at the various distributors, the prices for these things vary wildly, so I'd shop around.

--
Regards,
  Bob Monsen

God ever arithmetizes.
- Gustav Jacob Jacobi

it's not immediately obvious which transistors are PNP and which NPN (possibly due to the image resolution)

anyway it appears that you are switching the supply to the transistors that switch the motors on or off.

this is most unusual - and I'm not sure that it will work.

google for "H-bridge" and use one to drive each motor.

I think the device your original question was searchiing for is an inverter, (or "not" gate) these convert a "high" input to a "low" output and vice-versa.

Bye. Jasen

Here's a .gif of the H-bridge that was embedded in my circuit; I stripped off everything around it to simplify it and rearranged it a bit to make it look more like a traditional H-bridge, plus enlarged it so it's obvious which transistors are NPN and which are PNP.

I also drew the transistor swich I came up with, modified somewhat since it's conception last night/this morning.

Jasen: "I think the device your original question was searchiing for is an inverter, (or "not" gate) these convert a "high" input to a "low" output and vice-versa."

It took me a while, but I finally figured out how I could use inverters. I'd been confused with combining inverters with an NPN/PNP mix, but once I considered just inverters and NPNs the solution was obvious, and is presented (in simplified form) in the next .gif. It's simpler than the solution I developed for the NPN/PNP H-bridge, so I'll be using the inverter solution instead, but it's nice I at least got a

+Vcc/ground switch out of my confusion.

Thanks to everyone for their help.

yeah, I noticed that...

it depends on which logic family your and and or-gates are from, most will pproduce a pertty good path to ground when in the low state (the exception being PMOS which isn't real common)

you probably could, the bit with two transistors from the motor to the h-bridge output looks unusual... but it may work...

you should probably get some larger transistors instead of paralling them. junction devices don't always balance well, and with your low voltage supply adding series resistors to balance them could may wast too much of the voltage.

also using 8 500Mw transistors you could build two h-bridges.

oh... that's what you wanted, .

many chips come wired that way, it's called a totem-pole output in TTL chips it's implemented something like this: totem-pole output stage (from memory)

-----+---------+----- +5 | | | 50R 5K | | / | |/ |------| | |\\| in / ~\\ |/ | --| +----- out |\\| | ~\\ | | / | |/ `------| |\\| ~\\ | ---------------+------ 0V

Bye. Jasen

There are various subtle issues with regards to H-Bridges. For example, having a short delay between turning one side off and turning the other side on is good, because otherwise you will have a huge current zoom through the two momentarily connected transistors.

Also, it is often advantageous to run the motors at a higher voltage than the logic drivers. Your circuit, while nice, won't allow this.

Finally, as you may know, when you reverse a motor (or even shut it off), the inductance and mechanical inertia can cause a big voltage spike, which is often deadly to controlling logic.

These problems are usually dealt with by using an integrated solution. The H-Bridge I mentioned upthread deals with all of this (well, you will need protection diodes of course), and allows up to an amp of drive current... You can have split supplies, and run the motors at 12V, while running the logic at 5V so you can use a microcontroller to drive it directly. It has the 'inverter' logic built in, so when you turn on one side, it turns off the other for you after a short delay.

Even if you don't buy it, I'd look up the datasheet and look at the port schematics and protection schemes to get some ideas. Here is the TI page: .

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Regards,
  Bob Monsen

Man, as a social animal, can no more escape government than the 
individual can escape bondage to his bowels."

the right side has a current path from the top transistors emitter to its base then into the base of the bottom transistor... the transistorw will just burn up.

Also pushing the base much past the collector voltage will destroy a transistor. a resistor in series with the base will protect against this.

The left side is possibly workable, but the transistors might not turn all the way on and in that state they are inefficient and heat up.

you have the right hand transistor upside down.

your diagrams are hard to read, it's traditional to put the positive supply At the top and ground below. input on the left and output on the right.

redrawing your circuit (and flipping the backwards transistor)

------------------------+---+-------- +5V | | | | | | / | | / | |/ | +--| | | |\\ 68K | | \\| | | ~\\ | | | | +------+---+--[500R]--- out | | / | in | / | |/ | --[68R]-+---| | | |\\ 1MK iM | \\| | | ~\\ | | | | ----------+-------+------+-------------0V

that's not going to work. the top transistor will never turn on

this one shows promise (but the transistors on the right are upside- down again )

you'll need base resistors on the bottom transistors too, and the top onse probably won't turn all the way on...

(if the diagram looks crooked paste it into notepad or configure your news client to use a fixed-pitch font (eg courier))

------------+------------+-------- +5V | | +-------)------------)--[160R]--+ | | | | +--)-------}--[160R]----)------+ | | | | | | | | | | | | | | | / \\ | | | | |!/ \\|| | | | | |/~ ~\\| | | | +---| |--+ | | |\\ /| | / | \\ / | \\ | / \\ / \\ | A |/ | + | \\| B --[2K2]-+---| +----(M)-----+ |---+-[2K2]-- | |\\ | | /| | | | \\| / \\ |/ | | | ~\\ | / \\ | /~ | | | |/ \\| | | +-[10K]-+------| |------+-[10K]-+ |\\ /| | \\| |/ | ~\\ /~ | | ------------+------------+-------- 0V here there's two inputs A and B. putting either of them high will start the motor (opposite directions each) putting both high would turn all the transistors on and short circuit the power supply. (so don't)

Bye. Jasen