I need help with a basic circuit
 

I'm a CompE/EE student, but I haven't gotten into actually making circuits yet. Anyways, for FSAE, I "volunteered" to make a circuit for the car. Basically when the circuit recieves a 12v signal, it starts a timed delay, at the end of the delay, power is cut to another device. I was trying to accomplish this with a 555 timer and a d-type latch, but it doesn't work in the simulation software I'm using. Any help would be appreciated. Oh, BTW, I can't use relays, and the timer needs to have a variable delay.

what is the maximum delay that you will need?

This will tell you how to set up the inputs.

The input setup will tell you how to design your circuit.

0 to 60 seconds

how bout something like this?


the 6 control lines (C0-C5) allow you to make settings from 0 - 63 seconds (standard binary values).

12V input is constantly applied, and this go's through the circuit, which allowes it to pass through to the output for as long as the control lines are set higher than 000000.

Once the timer reaches the value passed in through the control lines, the voltage is cut off to the output, and the control is reset to 000000.

The circuitis then redy to accept a new control value, and start the cycle all over again


Does this sound like it would work for your application? If so, we can start working on the circuitry to drive it.

There is no reason why your idea would not work, is it possible that it may not be working due to simulator setting? Just out of curiosity what sim are you using?

Superart, your idea would work but having the timer controlled with a potentiometer would be more useful for me. As for the simulation program, I'm using multisim.

How would you use a pot for this?

Say for example, you set the pot for 3 seconds....
The timer kicks in, and passes the 12V along for 3 seconds.
After 3 seconds, the timer expires, but since the pot is analog, i is still set to 3 seconds, and the circuit is still set to read it. So instantly, when the timer expires, it is reset for 3 seconds.
so, basicly, what you get is a constant on situation.

Although I could be wrong. What does your sim say?


Just out of cureosity, what is this circuit being used for, and what school do you go to?

I go to UMR and the circuit provides an "idiot" function for the car, I'm not sure if I should post detailed info about it since it is for a competition and all. I don't think I explained the functionality of this circuit well enough, let me try again. The circuit has a constant 12v running through it but when it recieves a 12v signal it cuts the constant 12v after a timed delay. Also, my problem with multisim wasn't so much that it didn't work but rather it set the timestep to low and was opperating in milliseconds instead of seconds.

edit: the pot would be used in a circuit like this one http://circuitos.tripod.cl/schem/r84.gif

How does it turn on the 12v in the first place? Is the 12v signal used as a toggle to turn the 12v output on/off(with delay) ? This is easy to do with a toggle FF, diode, capacitor, some resistors, comparator, and a mosfet (since you can't use relays)

Is the 12v signal a seperate input.? Or is the 12v signal you are talking about the constant 12v feed.
If a seperate input is it a pulse?

This circuit doesn't turn on the 12v, it cuts a 12v output when it recieves a 12v input.

Can you just use a simple PIC chip?

Just to clarify our understanding:
1) You have a constant +12V power supply to the circuit and Ground
2) You have a +12V input signal to the circuit that will initiate a timer.
3) The timer, when complete, will interrupt a +12V line supplying power to another section of the overall design
4) You require some analog control over the length of the timer (simple pot, probably)
5) You require some method to reset the circuit, such as a reset button

To do this, you need several "inputs" to the circuit:
1) +12V PS
2) 0V GND
3) +12V SIGNAL
4) +12V INTERRUPTED LINE

Potentially, you could have separate grounds, but that is negotiable.

Mogwai is probably on the right track if you want discrete components. It's cheaper and faster if you know component design instead of microchip programmming. Simply use a switch to toggle a flip flop that initiates a current limited feed into a capacitor. The capacitor voltage is then compared to a reference voltage given by a pot (your timer control). When the comparison reaches "true", the comparitor sends voltage to a transistor which interrupts the +12V INTERRUPTABLE line.

There's more to a good design, of course. You'll want lots of signal conditioning if it is in an automotive environment. Also, pay good attention to your grounds! Do not skimp on components, as a blown transistor is a pain to replace on race day. Etc... etc...

Good luck.

so it has a consant 12v input going through it, and when it recieves another 12v 'control', it allows the original constant 12v to continue through for a specific length of time, based on the setting of the pot. Once the timer expires, the 12v 'feed-through' is cut.

Am I understanding this correctly?

If so, what do you want to happen once the timer has expired, and 12v 'feed-through' has been cut?

The state of the circuit at that point is, you have a constant 12v going into the circuit, but not being passed through. You have a 12v 'control' voltage set to TRUE, your pot is set to the preveous setting, and the timer is expired.


<edit>
hehe, crosspost.
:)

I was thinking about using a pic but I dont think I would be able to justify using one for such a simple task. Unless of course you add indicator leds, a countdown seven segment display, and maybe some audible feedback. Vector86, do you get extra credit for an over-engineered solution? :D

Well, a PIC chip can be a more compact solution, and when engineering time is more expensive than component costs (i.e. prototype), it works quite well. Just get a PIC that has an internal oscillator and you've gotten lots of stuff like comparitors and timers all inside the 8-pin DIP. I just think more in terms of SW solutions than HW discrete because of my background.

SW also has the advantage of being easier to modify the operation in the future.

You can use a uC with an ADC built in so you can use your pot

Microchip is very good with free samples. http://samples.microchip.com/
I like the pic16f88 from their mid-range series. I have used it with this boot loader: http://www.etc.ugal.ro/cchiculita/so...bootloader.htm

I don't think Vector86 knows much about microcontrollers though, since if he did he would not be asking how to do this. It took me a few weeks to read all the manuals, datasheets, application notes, learn the assembly instructions, and figure out how to use mplab and the simulator. Puting something together with descrete components would be faster than learning all that...BUT if you have the time, I suggest you do. Microcontrollers are amazingly useful. After I finished my senior project using some microcontrollers, I felt like I could build anything. :p

Microchip has TONS of docs on their stuff. Vector86, if you decide to go the uC route I can help you by pointing out the pdfs you should read. :)

You are probably right in thinking Vector86 isn't versed in uC designs.

As for PIC chips, they are great beginner chips. For advanced use, motorola and TI have some even better chips but they really require a bit more experience, because there isn't the same amount of community support for them. Some TI microchips have truly amazingly low power usages and still outperform PIC chips but they are kind of a pain to program. Etc...

Well, good luck, Vector86.

Yes

Sounds good, but I don't know how to implement any of this.

You are in deep doodoo, sir. I think you might have volunteered for something a wee bit over your head and might want to ask some of your team mates for help. Some people here might be able to suggest a design, but ... this is a school project, right? You shouldn't be getting credit for something you don't understand.

I did some reading on the PIC chip, it's way too complicated for this job. This circuit needs to be very simple. Brian256, I do have basic electrical knowledge, just not with some of the more advanced ICs. Another bit of info I should throw in: this circuit is a backup for the in dash unit that performs the same task, hence, it doesn't need to be as complicated.

Is this simple enough? When I needed a timer circuit in school I came up with this in about 5-10 min.

You do know how comparators, capacitors, resistors, and diodes work right?

That looks very simple, I could throw in a rheostat instead of that resistor to get the variable time control. What type of comparator did you use? I don't know what values to use to get the time I want though.

You can use an opamp as a comparator. For this application I would use a single supply one that can run on 12 volts. As for the pot, use it to vary Vr with a voltage divider.

Get a breadboard and play around with this stuff, thats how I figured it out!

Mogwai has a great suggestion. Get some of these parts, breadboard it up, and try it out. I think you'll learn a lot more than just trying it out on a simulator.

http://sombiri.syntechsoftware.com/M...er_circuit.jpg I breadboarded it up with an lm339 but I couldn't get it to work the way I want it to. Is there anything I should look closely at?