help with simple circuit, willing to pay for full design!

[seaneking]

Hi everyone,

A bit of background first, I know only the basics of electronics (ie: I know what components do and how to read schematics), but have no idea how to design circuits, hence why I'm coming here for help.

Now to the (slightly odd) circuit.

- In essence I simply need to trigger a DC motor to turn for a fixed amount of time.

- The switch that triggers the motor will be a push switch that may be only pushed briefly (not held down 100% of the time). The switch, once pushed, needs to be 'deactivated' so that pushing it again wont trigger the motor to turn again. I assume it then needs some sort of 'reset' button to activate the switch again?

- Also (yes, there's more) I need another switch that will reverse the direction of the motor so that it turns backwards for the same amount of time it was set to turn forwards in the first place. (no need for the whole deactivation/trip protection in this one)

- If the reset switch and reverse switch could somehow be one in the same thing it would be awesome, but really doesn't matter that much if I use two separate switches (so three in total)


I was thinking a 555 timer to control the whole motor timing thing, and a triac (SCR) to control the whole switch re-trip protection/one push triggering thing. How (or if) it could all fit together I have no idea. I would like to keep it as simple as possible, and I don't have the tools (or budget) to program anything on a microcontroller. I realize that I'm asking a lot, and of course I would love some free advice , but if someone is willing to design the whole circuit (schematic and all), then i would also be willing to pay (via paypal).

So, any ideas?

oh, and feel free to pm me if you'd be willing to design it for me/give me a quote.


Thanks!
Sean

 

[(*steve*)]

Trivial to do with a on-off-on dpdt switch with spring return from each position and a microcontroller. Oh and 4 mosfets to control the motor. I guess you could add a bit of logic to prevent both sets of mosfets being turned on simultaneously.

how many do you want

 

[seaneking]

The only problem being that I can't program a microcontroller lol

Is there a way for that same setup to work without the microcontroller? Also, the switch that first triggers the motor (the forward motion, trip protected one) needs to be a spst push switch for the application I have in mind. So I don't how that would work with the circuit you're describing.

Sorry if I've misunderstood, complete noob here

Thanks,
Sean

 

[(*steve*)]

OK, you could do it with 2 SPST pushbutton switches.

Yeah, you could probably do it without a uC, but it would be a *lot* harder

My suggestion would be to get an introductory PicAXE kit and learn to program it. There are lots of examples and you can program it over and over until you get it right.

 

[seaneking]

hmmm.... I might have to do that then. I actually own an old pic project book, it's just that I'm in my last year of highschool, so I don't have a whole lot of spare time on my hands with exams and all, and I'm taking off overseas next year so I didn't really want to invest time and money into something that I'm not really going to use any time soon.

But if there is no other solution, I might consider doing it....or just scrapping the project lol.

Thanks for all the help, and sorry if I'm being a pain

 

[Laplace]

Here is a concept design using just 4 chips. One dual monostable multivibrator with reset (CD74HC221), one quad NOR gate (CD4001), two power MOSFET pairs (IRF7389). Caveats: I have not checked and cross-checked the design nor built it on a breadboard; the Karnaugh maps are on scraps of paper, and the transition between positive and negative logic design can be mentally error prone; handling the timing components, power-on state, and open inputs are left as an exercise for the reader.

The monostable uses edge-triggered inputs which is convenient. Two of the NOR gates are used to remember when the FWD button has been pushed so pushing it multiple times will not activate the motor more than once. The FWD button is also gated to prevent holding down the REV button and then pushing the FWD button multiple times to trigger the motor multiple times. In the normal off state, both sides of the motor are connected to ground. Level shifters are used to drive the MOSFET gates so the motor voltage can be different from the logic voltage. The REV pulse is fed back to the FWD reset so that REV always takes precedence over FWD.

 

[seaneking]

That is awesome! thank you so much Laplace.

Just a few questions, because i don't completely understand it (I'm trying my best to get my head around this stuff).

First, how is the timing controlled? as you're using a monostable multivibrator I assume through the value of a resistor and capacitor somewhere, but for the life of me I can't see where. Unless it's controlled internally in the chip or something?

Also, am I right to assume that after the FWD pulse has been triggered once it cannot be triggered at all ever again no matter how often or when the button is pushed, until the REV circuit has been triggered?

 

[(*steve*)]

Laplace, I may be wormg, but the circuit you provided does not seem to limit the ON time of the motor, nor ensure that the reverse time is the same as the forward time.

 

[Laplace]

As I said, it is not the full circuit. You still have to add in the timing resistor and capacitor for each monostable. I was more interested in just the logic design. This is what the design process yielded. To find out whether it works exactly the way you want, either breadboard it or do a state analysis. Conceptual design is fun but detailed implementation is hard work.

 

[seaneking]

Okay, this is probably wrong, seeing as I have absolutely no idea what I'm doing, but would this work as a fully functioning circuit?

I'm guessing not, but I thought it was worth a shot.

 

[Laplace]

Also, I'm guessing not. Each monostable requires its own timing capacitor and resistor. Would suggest getting the CD74HC221 datasheet and studying that. There are other CMOS monostables available that might be made to work here, but that is an implementation issue.

 

[seaneking]

I already have read the datasheet, that's how I knew which pins were the timing input ones...I guess I missed the bit about independent capacitors and resistors.

So would this be alright?

Fingers crossed!

 

[Resqueline]

Separate is right, but, how'd you come up with that way of connecting Ct & Rt? It doesn't quite resemble the way it's done in the datasheet, have another look..

 

[(*steve*)]

...for each monostable.

Sorry about that Laplace, I didn't notice they were monostables

After thinking about this, it appears that the requirements so closely suit non-retriggerable monostables that it almost appears that the problem is contrived to cause answers to converge on this as a solution.

seanking, what is this actually for? I'm curious.

 

[seaneking]

okay, so I downloaded a different datasheet, which had the diagram of how to connect the capacitor and resistor. I hope I have it right this time.

As for the project....well, don't laugh but I stumbled upon this youtube video a while ago where this guy mocked up a shoe that supposedly tied itself, only it didn't really tie anything, is just pulled plastic laces around the outside, and there was a whopping great circuit on the back of it with two giant servos, and the laces were all over the place.

So I figured I could do better, I'm using an old shoe from home as a base to build a design I came up with where there is no circuitry/motors exposed, it's all inbuilt, and the laces look 100% normal, and the shoe is the same size, shape, etc. as normal.

In a nutshell, I'm making self tying shoes. (well, trying to make, probably going to epicly fail ). you know, just for the fun of it really.

 

[Laplace]

My Texas Instruments datasheet for the CD74HC221 Monostable Multivibrator shows each of the timing resistors connected to Vcc. Now if your datasheet shows the same thing, could you explain why you would choose to connect your R1a to a pull-up resistor and your R1b to a pull-down resistor instead of Vcc and then expect the monostable to work as its designers intended?

 

[seaneking]

oops! I didn't even notice, that was completely by accident, rather than by design. I'm really not good with circuit design, as you can probably tell, I'm more of a mechanical kind of guy

R1a should be connected before that resistor, and R1b should be connected to the other side of the reverse switch.... my bad.

 

[(*steve*)]

OK, another point. You have a diode in series with the power supply to the logic circuit (and presumably to the point where you take the power for the logic).

I presume that this is to decouple the power supply from spikes generated by switching the motor. You really should have a capacitor after it, and possibly a zener diode and small series resistance to limit positive going spikes.

 

[seaneking]

Okay so your suggesting I swap out that diode for a 5v zener diode and put in a small value capacitor in front of it?

so something like this?

And where should I put the resistor?

Sorry for all these questions, you guys have been so much help! Thank you!