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InvisibleI_was_the_walrus
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Timed switch to power a small motor
    #21291597 - 02/18/15 01:11 AM (8 years, 11 months ago)

I'm trying to build a small circuit board with an electric timer. When the timer goes off, I need to power a small dc motor that will pull some string and launch a rubber band trebuchet. I'm familiar with the basics of a board and simple components but thats about it. Anybody have a good idea on how I could throw this together?


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Invisiblekoraks
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Re: Timed switch to power a small motor [Re: I_was_the_walrus]
    #21291630 - 02/18/15 01:26 AM (8 years, 11 months ago)

A simple microprocessor circuit (e.g. arduino) would provide the most flexibility. But a simple plugin wall-wart timer and an off the shelf DC power supply would be the quickest fix.


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InvisibleI_was_the_walrus
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Re: Timed switch to power a small motor [Re: koraks]
    #21291668 - 02/18/15 01:40 AM (8 years, 11 months ago)

Its gotta be small and light enough to fit on a modified rc type car. I've actually built something similar to that (basic breadboard), but Im still not sure on how to incorporate and exploit the timer.


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Invisiblekoraks
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Re: Timed switch to power a small motor [Re: I_was_the_walrus]
    #21292115 - 02/18/15 06:05 AM (8 years, 11 months ago)

I would recommend a small microprocessor in that case. An arduino would be the easiest to implement, but if it's too big physically, you could consider a circuit around e.g. an atmel attiny, those are pretty user friendly as well, but it requires a bit more work.


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InvisibleSun King
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Re: Timed switch to power a small motor [Re: I_was_the_walrus]
    #21292194 - 02/18/15 06:56 AM (8 years, 11 months ago)

What kind of timer? Do need a delay or a 24 hour clock?


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InvisibleI_was_the_walrus
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Re: Timed switch to power a small motor [Re: Sun King]
    #21293325 - 02/18/15 12:15 PM (8 years, 11 months ago)

I need a 10-20 second delay. I was thinking a mechanical kitchen timer could be used to close a circuit thus powering the motor but it would be hard to time as they only come in 1-60 minute intervals. I read that a digital alarm could also be used. The timer goes off thus powering the alarm speaker which is rerouted to a motor, but the line would only put out mV. Maybe theres a way I could tie in a 9V battery to power the second motor after the timer goes off?


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Re: Timed switch to power a small motor [Re: I_was_the_walrus]
    #21293345 - 02/18/15 12:19 PM (8 years, 11 months ago)

If alls your doing is launching a trebuchet why not use a servo which you could tie into the car wiring and be able to operate the servo via button on your radio.


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InvisibleI_was_the_walrus
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Re: Timed switch to power a small motor [Re: Mental Taco]
    #21293486 - 02/18/15 12:49 PM (8 years, 11 months ago)

The vehicle has to be totally self propelled and guided (its for a school project). The idea is to go a certain distance, stop, then fire a projectile at a target (all on the cheap btw). I've got the basics laid out with most of the components wired up and working but the whole stop and then fire on a timed delay thing has got me looking for other options. I figured some kind of trebuchet/tension loaded weapon would be easiest. That way I could just power up another motor set on a timer to pull some wires/release the projectile.


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InvisibleSun King
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Re: Timed switch to power a small motor [Re: I_was_the_walrus]
    #21293614 - 02/18/15 01:16 PM (8 years, 11 months ago)

http://en.wikipedia.org/wiki/555_timer_IC

A 555 timer chip can introduce a delay in one-shot mode. You could press a button then 10 seconds later it can trigger a relay. I am not sure how long a delay you can get, you could cascade them if necessary.


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InvisibleSun King
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Re: Timed switch to power a small motor [Re: Sun King]
    #21293660 - 02/18/15 01:25 PM (8 years, 11 months ago)



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Invisiblekoraks
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Re: Timed switch to power a small motor [Re: Sun King]
    #21296622 - 02/19/15 12:53 AM (8 years, 11 months ago)

Several options are possible, including the mentioned 555 or just an RC circuit, but I still think a microprocessor will turn out to be the best option in terms of footprint and flexibility. Plus they are quite cheap these days. And you can use it to control some other stuff as the need arises.


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InvisibleBacchus
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Re: Timed switch to power a small motor [Re: koraks]
    #21329873 - 02/25/15 07:04 PM (8 years, 10 months ago)

koraks' advice is very good, and that's the way that I'd go. If you wanted to, though, you could use a digital kitchen timer and tie into the speaker supply to bias an SCR. Depending on the timer, it take some jiggering and some pokering to get it working, but it would be a fun way to learn about circuits. If you want to go the RC/555 route, google "egg timer circuit"


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OfflineSeussA
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Re: Timed switch to power a small motor [Re: Sun King]
    #21333685 - 02/26/15 03:30 PM (8 years, 10 months ago)

> A 555 timer chip can introduce a delay in one-shot mode. You could press a button then 10 seconds later it can trigger a relay. I am not sure how long a delay you can get, you could cascade them if necessary.

This was my initial thought.  Might even be able to get by with a simple RC circuit to the gate of a mosfet.  As the capacitor charges up, it will eventually switch on the mosfet.  Not as accurate as the 555, but a smaller BOM and very simple.

>  A simple microprocessor circuit (e.g. arduino) would provide the most flexibility. But a simple plugin wall-wart timer and an off the shelf DC power supply would be the quickest fix.

Way too complex for something like this.  Yeah, it will work, but why spend the money and add the complexity when 10 cents in hardware will do the job?


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Invisiblekoraks
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Re: Timed switch to power a small motor [Re: Seuss]
    #21336366 - 02/27/15 01:34 AM (8 years, 10 months ago)

Because it's more flexible, it is easier to modify by changing the software if the functionality must be changed in any way without the need to change the hardware and it's actually not much more complex than a 555 circuit in terms of hardware, neither is it that expensive since a small attiny costs about a dollar.


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Re: Timed switch to power a small motor [Re: koraks]
    #21338101 - 02/27/15 12:09 PM (8 years, 10 months ago)

If you're going with a 555 circuit want the timer to stop the vehicle, and then fire the rubber band some number of seconds later (instead of simultaneously), you can use the 555 in astable mode and have it clock a 4017 decade counter.

This means that you can have the 555 put out a short pulse every, say, 2 seconds. That signal feeds into a decade counter's clock input. The counter has 10 output pins and counts with each clock pulse like this:

Code:
Q9........Q0
"0000000001"
"0000000010"
"0000000100"
"0000001000"
etc.



You can have a high signal on Q7 or Q8 cut the power to your car's drive motors, and then the have a high on bit Q9 fire the rubber band.

OR you could have two monostable 555s. One of them cuts the power, and then starts the second one which fires the rubber band.

There are a lot of ways to skin this cat.


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Edited by Bacchus (02/27/15 12:17 PM)


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InvisibleI_was_the_walrus
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Re: Timed switch to power a small motor [Re: Bacchus]
    #21339353 - 02/27/15 04:52 PM (8 years, 10 months ago)

Maybe something like the traffic lights circuit?

http://www.555-timer-circuits.com/traffic-lights.html

Instead of 3 LEDs (2 volts each?) is there a way to power 2 motors at 9-12 volts each, one at a time?


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Re: Timed switch to power a small motor [Re: I_was_the_walrus]
    #21339441 - 02/27/15 05:15 PM (8 years, 10 months ago)

Yeah, just replace the LEDs with either switching transistors or relays (with flyback diodes!) Not sure how you intend to use that traffic light circuit, though.

You're not going with a microcontroller?


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Invisiblekoraks
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Re: Timed switch to power a small motor [Re: Bacchus]
    #21341330 - 02/28/15 01:58 AM (8 years, 10 months ago)

I'm not sure how the traffic light circuit is very relevant; you're looking for something like this if you want to use a 555: http://www.electronics-project-design.com/timedelaycircuit.html


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Re: Timed switch to power a small motor [Re: koraks]
    #21385719 - 03/10/15 05:28 AM (8 years, 10 months ago)

> Because it's more flexible, it is easier to modify by changing the software if the functionality must be changed

Guess I'm looking at it from a mass-production standpoint where pennies matter to the profitability of the project. 

Even at the hobbyist level, I find it easier to bodge in a new resistor/capacitor than to break out the computer, re-write a bit of code, compile it, find my programmer, find the proper cable, try to remember which orientation on the header to plug it in, download the new code, test it, etc... flexibility comes with a lot of complexity for something that can be done with a handful of jellybean parts out of my parts bin.


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Invisiblekoraks
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Re: Timed switch to power a small motor [Re: Seuss]
    #21387214 - 03/10/15 02:08 PM (8 years, 10 months ago)

Well, even from a cost perspective, a small micro can be a better solution if significant volumes are going to be produced. I talked to a friend last month who works at a tech company and they had an issue with an sd card that needed a delay in power off in order for the file system to complete its tasks. So I said what you suggested: you must have used a simple RC circuit to create a delay. Turns out that an R and a C combined was more expensive (particularly the C, given its needed size; also physical space was a limitation btw) than a tiny microprocessor. I wouldn't have guess, but sometimes, the optimum solution is a bit counter intuitive. Of course, in a hobby project, personal taste is much more of an important factor. I admit that I would sooner than you grasp for a micro for the reasons I mentioned, which certainly isn't to say it's the best solution for the next person!


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