Wednesday, October 6, 2010

Chapter 5 - Exercise 32 Update 3


Before cutting out the wood for the shell of the robot, I figured it might be wise to actually wire up the schematic first and see if I can get all the electronics to work. As you can see from the video and picture, I've managed to wire up the motor - I've got a temporary push button that allows me to provide power to the circuit. I'll get an actual on-off switch once I get the circuit de-bugged.

The circuit is definitely providing power (6V) to the motor, but the two microswitches are not working as desired. When one is pressed, it's supposed to drop the voltage on pin 2, triggering the coil and reversing the spin on the motor... but that's not happening. Pressing either switch does not reverse the motor. I'm using different microswitches than the ones seen in the chapter, but I don't think that's the issue.

I also need to get out my chip tester and make sure that voltage is detected on pin 3 that runs to the coil.

Obviously, any suggestions my readers may have will be appreciated.

12 comments:

  1. It looks like there's something else also connected to pin 2 on the IC. When the switch closes, it may not be getting a proper connection to ground, and therefore not making the right comparison.

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  2. Yes, pin 2 not only is connected to the switches but it also is connected to the 10K resistor - I think that's what you're seeing in the photo - I've had to run it up (instead of down) and my breadboard layout doesn't match the schematic in terms of positioning. Take a look at the schematic and you'll see that pin 2 has quite a few connections it shares with pin 4, pin 8, and pin 7 (I believe).

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  3. Somehow I ended up here from twitter. I took a look at your schematic and it looks bogus. From the schematic, I'm guessing that's a LM555 IC. It's been a long time since I looked at one but I believe pin 2 is the trigger input, pin 3 is the output. The IC needs to turn on and off the relay in order to reverse the motor direction. Both of your switches are in parallel so pressing either one will do the same thing, pull pin 2 (trigger) to ground.

    Are you expecting a temporary motor reversal?

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  4. j,

    You've described exactly what the circuit is supposed to do - and I've double and triple checked the wiring of the switches and the 555 timer chip. Everything looks okay, but I'm sure I'm missing something small. The relay is also something I'm a little unsure of... I'm sure I've wired it correctly but I'm wondering if it might have become damaged at some point... but the buzzing is the arm vibrating as it turns off and on, right?

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  5. Oh. I think I get what you want to do. The two switches are for a bumper or something. So once a switch activates, how long do you want the motor to reverse? What are the values of the capacitor and two resistors connected to pins 6 & 7?

    As for the buzzing, I turned my volume up and the only thing I heard was the motor vibrating on the table.

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  6. That is the old video... I'll try and shoot a new one tonight.

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  7. resistor & capacitor values?

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  8. Also, do you know how much current your relay takes? I'm not sure if it's a big industrial relay or a small micro relay. The 555 can only supply a max of 200mA. One thing I don't like about the circuit is that it doesn't include a shunt diode across the relay. When the relay is turned off, there is a spike of current generated from the relay because really it's just an inductor. Google for 'relay protection diode'. You will see a diode that is hooked backwards across the coil of the relay.

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  9. I'm going to post the schematic and an updated video later tonight - but you've got me interested in the idea of inserting a diode before the coil... I assume you're talking about a diode that allows current to flow only out of pin 3 and into the coil?

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  10. It's a diode that goes in the reverse direction across the coil of the relay. Following your schematic, the anode would go to gnd, and the cathode would go to pin 3. Only the flyback current from the coil would flow through the diode. While trying to find an example on the web, I found this discussion that possibly describes your problem: http://www.electronics-related.com/usenet/basics/show/67253-1.php

    Here's a link that shows a shunt diode for a 555 driving a relay: http://www.555-timer-circuits.com/driving-a-relay.html

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  11. BTW: I found a decent description of your design here: http://www.scribd.com/doc/9157793/The-555-Timer

    Look for the description describing 'MONOSTABLE MULTIVIBRATOR'.

    You will also see the circuit where the output drives an LED. If you have an LED, it might be an easier way to debug your design. You will need to choose the correct resistor value that goes in series with the LED. It will be dependent on the type of LED and what your +V is.

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