Wednesday, October 20, 2010

Chapter 5 - Exercise 32 Update 5

Per Retrophile's suggested tip involving my relay being wired incorrectly, I changed the wiring and figured out my mistake (thanks Retrophile) - I assumed incorrectly that the relay's pins fit logically to the pin layout in the schematic... they don't. Going to the documentation for this particular relay verifies that Retrophile's pin layout is correct.

The two photos here show the rewired circuit - same circuit but just a slight change in photo angle for you to get a better look.

So, I've rewired the relay and as you can see in the first video below, the motor spins but the switches are still not causing the motor to stop and reverse. I used my multimeter to verify that I've got the switches wired correctly - the top and bottom pins are Normal Open (using the middle pin and bottom pin is Normal Closed and the multimeter shows a 1 when the button is NOT pressed... 0 when it is pressed). Per the schematic, we want the switches open, so I'm fairly sure I've got that wired correctly. I also replaced the 555 chip with a new one, just in case... but no luck.

So, my next step was to shoot another short video showing me using the logic probe on the 555 chip. My description here may not be 100% on the money, but as I understand the chip and the schematic, pushing a switch causes pin 2 to detect a drop in voltage... and causing pin 3 to allow current to flow (or is it just an increase in voltage - is voltage always present? Something to look into...)

As you can see from the video, putting the logic probe on pin 3 and pushing the power button causes the probe to change in pitch... that should be right. But then when I press any switch, pin 3 should change in voltage and be detected by the probe... but nothing happens. Any thoughts?


  1. I assume from the lack of comments and a more recent update that you are still having problems.

    Checking your latest pics it appears you still have problems with your wiring. See for details on how it looks to me.

    Other comments and hints on what to check for are as follows.

    Switches -> Unknown pinout but they look consistent so I am assuming they are OK. To prove this use a multimeter on 555 pin 2 and ensure that the level is normally high and goes low when either switch is pressed.

    50K Potentiometer -> Unknown pinout and unclear wiring but although it looks backwards to me that wouldn't stop it working. A multimeter check on 555 pins 6 or 7 should show a +ve voltage which changes in value as the pot is altered. This is used as a timing adjustment and will affect how long the motor reversal lasts.

    Relay -> Still not right!

    #Firstly, the link between pins 6&9 appears to have missed 9 and landed in limbo although looking at the video it looks like you might have caught that one.

    #Secondly, pin 16 is still not connected. This should be connected DIRECT to 555 pin 3. As I pointed out in my previous comment the diode currently connected to 555 pin 3 appears to have missed pin 16 and ended in (pin15) limbo anyway.

    In any event the diode SHOULD NOT be inline with this connection. If you still want to use it check out the links given in previous comments by me and others as to why it might be wanted and how it should be connected (between relay pins 1&16)

    555 -> I couldn't see any note on why you switched to using 2 ceramic caps, apparently in parallel, on pin 5. I assume that adding the two values together still gives the desired value although this would only affect the reference voltage anyway and have negligible effect.

    More significantly, I cannot see where the 47uF cap is connected. In the latest picture it originally appeared to be connected to pin 5. Although I now think that was just an illusion I still can't see if it is correct or not. This works in conjunction with the 50k pot to set the reversal time and must be between 0v and pins 6/7.

    Pins 6&7 will normally be high. When pin 2 is taken low by either of the switches they will gradually fall in voltage until they reach approx half the supply value. They will then jump back to their normal high value. The fall time will continue irrespective of when pin 2 is released. (If the switch is still pressed at the end of the time period it will simply start another time period.)

    Pin 3 will normally be low. Once pin 2 is taken low by either of the switches it will immediately go high and stay high while pins 6&7 are decreasing. Once pins 6&7 return to their normal high value pin 3 will go low again.

    The time taken for the reversal period is set by the 100K+50k resistors and the 47uF capacitor. I couldn't be bothered to calculate the time period but a quick mock up seems to give a value of 5-15 secs reversal time.

    Finally, as a tip for future breadboarded projects, if you fit the power switch in between the input and the onboard +ve rail then connecting everything will be much simpler without the need for extra switched power links everywhere.

    I hope this was of use and that you manage to get this project working. Do let us know what the problem was once you get it fixed. - Good luck

  2. Thank you, Retrophile! Great list of things to check... I'll print it out and use as a checklist to debug.

    Email me at when you have time... would like to discuss an idea with you.

  3. This comment has been removed by a blog administrator.