Thursday, March 18, 2010

Chapter 4 Experiment 20 - Time to Move On

Okay, so I took some time away from this experiment, hoping that I'd be able to come back, find my mistakes, and get this circuit working. Well... no luck. But I'd like to try and explain a few things I did differently this time and a few things I was able to verify about the circuit - maybe this will help someone else trying to get this crazy thing working.

First, I totally rewired the entire thing. I found two errors in my wiring on the first try. This time around, I wired up each chip, double and triple-checked the wiring before moving on to the next chip. This worked well, and I was able to get the * button working and lighting up the LED.

After completing the wiring, the # sign would work and light up the LED... but this time I couldn't get the * button to light up - I took a voltage reading - I was only reading 1.0 volt across the LED - I tried different LEDs but no luck. I also tested my LEDs as the # button LED and they worked, so they obviously weren't burned out. My only thought is that they're simply not getting enough voltage. I think I'm using low voltage LEDs but I may be wrong. Either way, why would it initially light up but now now? And it doesn't matter anyway - even if the LED doesn't light up, pressing the * sends current to pin 8 on the 7408 chip, turning it on. The LED is just a visual cue.

Okay, so scratch that... next thing I wanted to figure out what whether the 555 chip was sending any voltage to the LED when the * button is pressed... remember, you have to hold down the * button while entering the code... when I pressed the * button I got a 0.47 volts reading on my multimeter. But isn't pin 3 only supposed to send a pulse if voltage is dropped on pin 2? If I'm detecting voltage on pin 3, this would mean that the 7408 is sending a 1 to pin 3 on the 7404 (a NOT operation) which in turn sends a 0 (no voltage) to pin 2 on the 555 chip. Which means I'm entering the code correctly... so why isn't the LED lighting up? One of the pics shows me using my multimeter and getting a voltage of 0 on pin 3... I didn't have enough hands to hold down *, enter code, snap picture, and hold the probes... argh... so you'll just have to trust me that I got the .47 volts on pin 3...

Oh, well... I gave it a second try, but I think it's time to move on to Exercise 21.

Oh, and there's an unlabeled resistor in Figure 4-84 (I'll submit to errata if it hasn't already been done) - I think it's supposed to be a 10K to match the 10K resistors for the 1 keypad button and the 4 keypad button... I may be wrong.

So, this concludes my attempt at Exercise 20. When I'm done with the book, I may come back and give it one more try... because I hate to leave an exercise with doubts. Is it working? I think so... but I can't get an LED to verify... but I do get positive voltage on pin 3 when I hold down * and press 1, 4, then 7.

So... this leads me to my next contest. I'd like to reward a Maker's Notebook to the first reader who can upload a video showing this circuit working in all its glory... it doesn't have to be mounted to a computer, but it should show all the LEDs working as desired - it should show an improper code being entered as well as the correct code and all the LEDs lighting up as they should - especially the * sign as you hold it down and the UNLOCK LED when the code is entered. First person to post a video that clearly demonstrates the circuit working gets the Maker's Notebook. (Sorry, Charles - as the author, you're prohibited from entering the contest... )

I've got a few videos below to finish up and then it's on to Exercise 21... thanks for sticking with me folks - got a bit behind in "real world" work and had some family business that had to be attended to... but I'm caught up and ready to get back on track.


  1. Hey James,

    I've been following your blog for the last several weeks since I've been doing these projects in almost parallel time as you.

    This experiment definitely gave me problems as well. On my first build I found that no matter which order I pressed the 1, 4, or 7 buttons the relay would switch. I think I had a few other bugs as well. I must have triple checked the wiring and I honestly could find nothing that didn't match the example in the book. In frustration I yanked all the components in disgust.

    After about 3 hours I couldn't help but go back for a second try. This time, I laid out the major components, but when it came time to wire up the various pins and leads I stopped following the book religiously and just worked through the logic of the circuit (as intended in the book). This time when I supplied power to the circuit everything was working as intended.

    I can't remember the exact difference in my circuit from the books circuit, but there is definitely something off about the example circuit. If I have time (and it would be helpful to you), I'll try to rebuild it to its working state, and provide better information on how it needs to vary from the example.

    This is probably something that will need to be covered in errata.

  2. Ryan,

    I also felt like I got the circuit to work... partially. I think I got too focused on trying to get the various LEDs to light.

    If you attempt the experiment again, please let us know what you find... and anyone else who attempts it. I'd definitely like to see a video of the circuit working.

  3. Is the contest for a Maker's Notebook still open? If so, I've just done this experiment, and I've uploaded a video showing it working.

    Experiment 20

    If you want to see the circuit in higher definition, I've uploaded a picture of it here:
    Hi-Res Picture of Circuit

    I also made a small modification to the circuit, shown in this video:
    Small Tweak

    1. Thanks for sharing the video and picture, Stephan. I had issues with the relay as the output from my 555 was lower than the input 5V to it. So I used a pull-up resistor from pin 8 to pin 3 to get it to deliver 5V and trigger the relay. From your picture, it seems you used a pull-up resistor to the relay input. Did you do it with the same intent of increasing the voltage to the relay in order to trigger it?

  4. I also had issues with this circuit. My 555 was working, but pin 3 did not achieve the full 5V from the input (what is expected). Without full 5V, the Panasonic DS2ESL2-DC5V relay would not trigger at all. In this case, I had to add a 1K ohms pull-up resistor to the 555 timer (from pin 8 to pin 3).

    In order to get it to work, I'd suggest you forget about the keypad initially. Connect the "*" wire to positive to simulate that it's pressed all the time, and have the wires for 1, 4, and 7 touching the positive input when you want to test the circuit. That way you avoid having to deal with the keypad, with loose wires, with pressing the "*" to get the system to test while you're checking the multimeter, etc.

  5. I had the same issue - the output from the 555 wasn't sufficient to trip the relay. I tried the solution posted by Rodrigo, but was only able to get it to work with a smaller value resistor - and then the indicator LED wired to pin 3 of the 555 was always on dimly when * was pressed. So my solution was to ass a 2N2222 transistor, with the 555 pin 3 going to the base through a 10K resistor, then +5V -> relay coil -> collector and emitter to ground. I also put the indicator LD and resistor in parallel with the coil. Worked like a charm.

  6. I did this project, works good, what did right out of the was to follow de Castro advice and skip the keyboard connections, just put a tactile switch for now. Another thing is the output of 555 made the relay work but noticed the LED would not stay on for that 1 second. I just added a transistor and worked like a champ.

    My question if enyone knows is can you show me a semantic on how to connect the 555 under ENHANCEMENTS so don't have to hold the asterisk on? I tried with no luck, thanks.

  7. Here is a video I made Watch "Make:electronics Experiment 20" on YouTube
    Make:electronics Experiment 20:

  8. I've worked on this without success. None of the above solutions work for me. I've noticed that the videos of people having it working use different relays--maybe ones that do not need 5V. The voltage on 555 output pin 3 will be up to 1.7V lower than the input voltage on 555 pin 8, or so I've read in several places, including Charles Encyclopedia of Electronic Components Vol. 2. John Cantalupo's explanation makes sense, but does not work, at least the way I have it wired--it's probably me. In any case, I have 4.86V going into pin 8 and 3.77 coming out of pin 3 between triggering and reset via threshold. That seems to be the way the 555 is designed. Am I missing something? See for detail and video (that's one of 3 posts on this experiment). Everything else works as expected.

    1. I submitted an erratum on the book regarding the voltage drop on 555 pin 3. Charles' response: "I think you're right but I am traveling right now and do not have a copy of the book. I will try to address this soon."

    2. It works. Following John Cantalupo's advice, I added a transistor. It took a while, because I'm dumb and because I cooked a couple or transistors. Once I was absolutely convinced I had it wired correctly and it still didn't work, I changed transistors, and it worked. Summary: the 555 won't put out enough voltage, the solution is to add a PNP BJT transistor like 2N2222A with emitter to GND, base to 555 pin 3, and collector to the - side of the upper coil on the relay. The + side of the relay coil goes to the 5V rail. So, when the 555 is triggered, the voltage on the output pin that is insufficient to trip the relay is enough to activate the transistor, which allows the 5V to pass through the relay to the collector and on through the emitter to GND. See this video: and

    3. sorry--that's NPN transistor not PNP