Monday, March 30, 2015

@MAKE Electronics Experiment 23: Nice Dice

This is an interesting and challenging experiment. Challenging because I sometimes have problems getting from schematics to breadboards, and I always have in the back of my mind that I may have cooked a chip.  No cooking this time, just stupid wiring tricks.  But, I got it to work.

We started with a 555 timer to send pulses to the a 74LS92 counter chip and 3 LEDs to count from 0-5 in binary, Note that the LSs are TTL chips as opposed to the HC CMOS chips we've been using.

Since the idea is to emulate dice, we need 7 LEDs, one for each dot on a die. We did that by adding a 74LS27 quad-gate triple input NOR chip, along with 4 diodes too protect inputs from flowing back into outputs.  In this version we had low-current LEDs tied to GND through 4.7KOhm resistors. This LEDs were not too bright (like me). By connecting the middle dot, to one output. each of the to diagonal pairs to 2 more outputs, and the two middle side LEDs to another, we were able to produce 7 combinations from the 4 outputs by connecting thim. The middle lights on 1, 3, and 5.  The middle sides light on 6 only.  One diagonal lights on 2, 3. 4, 5, and 6, and the other lights on 4, 5, and 6.

Next step was to add an inverter.  In the previous set up, the NOR chip needed to power the LEDs, What we want is for it to sink power, but to do that we needed to reverse the logic.  We used normal LEDs connected to power through 100Ohm resistors, and connected the inverter outputs to the negative side if the LEDs.  This was much better, because the LEDs are much brighter. 

Making the right connections from the NOR to the inverter to the LEDs turned out to be a challenge, but I stuck with it.

Finally, I replaced the 10uf capacitor between pins 7 and 8 of the timer with a .01uf, making the lights flash 1000 times faster and the change not visible (to me, at least).

So, you hold down the pushbutton and all LEDs appear to be lit. Release the button, and you have a roll of the die. Nice dice

Saturday, March 21, 2015

@MAKE Electronics Experiment 22: DeBouncing and Flip-Flop

This experiment is deceptively straightforward. The only issue I had was a bad chip.

First, I mentioned in the last experiment that the slide switch was not breadboard friendly.  The pins are not long enough and with only one row of pins it's not stable. I tried taking it apart with the intention of making a breakout board, but that proved to be harder than it should be (I should have left it together).  So, I went to Radio Shack and found this. The advantage is that it's a DPDT and  has 2 rows of pins, making it more stable.

So with a better switch, I tackled the experiment. Both circuits do the same thing. Debouncing means ignoring errant button pushes. Since latching logic gates set in the first impulse, anything further is ignored.  We latch by feeding the outputs of each gate to one of the inputs to the other.  The second input is connected to a pull-down (NOR) or pull-up (NAND) resistor and to one side of the switch. The pole of the switch (we only use one) is connected to Vcc (NOR) or GND (NAND).
Each output also powers a low current LED when it's HIGH.

NOR output is LOW unless both inputs are LOW.  When the switch is towards a gate's input, that input goes H, making the output  L. That gate's LED is off, and one input to the second gate is L. Since the switch is away from the second gate, that input is also L, making the output H, turning on the LED and making one input back to the other gate H, keeping it's output L and it's LED off. Switch to the other gate and one input goes H, making output L  and both inputs to the other gate L and it's output H and LED on.  It can only be in one state or the other. Debounced.

NAND works similarly.  NAND output is H unless both inputs are H.  The switch is connected to GND and each gate has 1 input connected to a side of the switch and a pull-up resistor. So, one input is H unless the switch is closed to that side, in which case it's output goes H,turning on the LED and feeding H to one input of the other gate.  Since the switch is open to that side, it's pull-up resistor makes the other input H, making the output L, turning off it's LED and feeding L back to the non-switch input of the other gate, keeping it's output H until the switch is moved (or power to the circuit is cut).

Fun and interesting. Here's the video.

Tuesday, March 17, 2015

@MAKE Electronics Experiment 21: Game Show Button Controller

After the last experiment this one is pretty straightforward. One 74HC32 quad OR chip, 2 timers, one SPDT switch, 2 tactile buttons, 2 LEDs, 3 10K resistors, 2 330 Ohm resistors. The wiring's the thing.

I couldn't get it to work at first, but it was just a loose connection. That, and my "breadboard-friendly" SPDT slide switch wasn't so breadboard friendly. I had to bend up the tabs on the side to get it to fit the breadboard at all, and the legs still weren't long enough. But it works.

Anyway, the switch is for Art Fleming to activate and deactivate the contestants buttons.  The buttons are tied to Vcc through a pull-up resistor. A jumper from one side of the switch to the side of button 1 1 not connected to Vcc and then a second jumper from there to the corresponding button on button 2. The other side of B1 goes to OR Gate1, input 1. The same for B2, the corresponding side goes to OR Gate 2, input1.  Both input2s are connected to Gate3 output. Gate3 inputs are connected to the output pins of the 555s.

So, in order for the output of either 555 to go H, the input on trigger pin 2 must be L. The trigger pins are tied to the outputs of OR Gates 1 and 2.  OR output is H if either input is H.  One input is tied to 555 output, which is L until triggered by it's corresponding OR Gate. Button outputs are H if no action is taken, so the 555s are not triggered. When a button is pushed, the voltage from the button goes negative, making both inputs L, and the corresponding output L, thus triggering the corresponding 55 output. Both 555 outputs are connected to LEDs, so the LED lights and stays on until reset.  Once one 555 output is H, Gate 3 output is H, and neither button has any effect because there corresponding outputs will be H.

When Art Fleming activates the contestants buttons and asks a question, the first contestant to press the button lights his/her LED and locks the other contestant out.  The other side of the slide switch is connected to the 555 reset pins (4), so when Art slides back the LED turns off.  When he's ready for the next question, he flips the switch back and the cycle repeats.

Here's the video.

Sunday, March 15, 2015

@MAKE Electronics Experiment 20: Keypad Security System Epilogue Part II: Making it Work

Getting past the fact that the 555 won't supply enough voltage, ever, the next question is how can we amplify it. The answer is with a transistor.  It took me way longer than it should have to get it to work, because of wiring problems and cooked transistors.

First the wiring:  instead of taking 555 Output Pin 3 to the + side of the relay coil, I connected it to the base pin of a 2N2222A transistor (NPN BJT).  I then connected + side the relay coil to the 5V rail, and the - side of the coil to the collector pin of the transistor.  The emitter pin goes to GND. Initially, I left the LED in the circuit, but since that's connected to GND, the circuit was always completed. I'm sure there's a place I can put it, but it works without it.

After few wrong connections, I still could not get it to work. I was absolutely sure that it was wired correctly.   During this process I must have cooked a transistor or two, In desperation, I replaced the transistor again, and it worked.

So, that's the answer:  insert a transistor, properly wired, and the voltage that was too low to trip the relay is enough to activate the transistor, allowing 5V to flow from the 5V rail through the relay to the collector, and when the 555 is triggered, the output pin will supply enough voltage to the base pin of the transistor to allow current to flow through to the emitter, completing the circuit.

Here's the video

I'm glad I got this to work.  I hate to leave something incomplete..

Saturday, March 14, 2015

@MAKE Electronics Experiment 20: Keypad Security System (Epilogue--May be the Book's Fault!)

As I noted that 555 Output Pin 3 voltage will be lower than the Vcc on Power Pin 8 by up to 1.7V.  That probably should have been the first thing I checked, but...  Anyway, I was puzzled by this--how can we flip a 5V relay if we design a 5V input into a chip that cannot put out 5V.  I went the book page on the O'Reilly website and got this response:

Your Errata Submission for Make: Electronics


11:53 PM (10 hours ago)
to meplattland
Hi Virgil Machine,

Thank you for submitting errata! The author of Make: Electronics, Charles Platt, has written you the following 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.


We appreciate the time you took to write.

Kind regards,
O'Reilly Customer Service
O'Reilly Media, Inc.


Your Errata Submission:

Type: Serious technical mistake
Page: 200
Location: Questions, 1st paragraph
Many have had trouble getting the relay to activate due to insufficient voltage. This paragraph says that the reason for using a 555 was to deliver enough voltage. However, according to Charles' Encyclopedia, Vol 2, the voltage on the output pin will be up to 1.7V less than the input.  Since input is 5V, and that's what the relay needs, how can that be? In my case, I'm giving 4.86V to pin 8 and getting 3..77 volts on pin 3.  Is this an error or am I missing something?

So, this may be a dead end.  I have to figure out how to increase the person reports having success with a transistor, but I can't get that to work--I may not be understanding his wiring directions. More learning to do.

Friday, March 13, 2015

@MAKE Electronics Experiment 20: Keypad Security System (Part III--still not working)

This has taken a while. Snow, family visiting, setting up my new generator, an fuel oil spill in my basement, and other events have dominated my attention.

When last we left this, I was convinced that the keypad was sucking current from the circuit. I used this one.  The only videos I've seen of people getting this to work use momentary buttons instead of the keypad, I tried to substitute buttons--that didn't help. Testing various locations with my meter, I see that's not the problem

I checked the voltage coming into the first part of the circuit, and then then through the operation. I put a meter set to continuity connected to the relay. If the upper coil on the relay gets 5V, there should be continuity, and 5V to the lower coil should reset it. As you can see in the video, holding the '*' key and entering the code causes the indicator light to turn on for ~1s as designed, but the relay does not flop.  The voltage meter shows <4V coming out of the 555--not enough for the relay. If I hit the relay with a jumper wire connected to the 5V rail, the relay flops and there is continuity until I press the '#' key or hit the lower coil with 5V,

Testing various locations with my meter, I see that's not the problem.  Here's the video.

The 555 has 4.86V going in on pin 8 and only ~3.7 going out on pin 3.  That's the problem, and I don't know what to do about it. I fussed with capacitor and resistor values with no change.  For the video, I wound up with ad 1000uf capacitor and a 10K resistor, so pin 3 would have output for longer and I could see it on the meter. I tried various combinations of 2.2, 10, 100, and 1000uf capacitors and 1K, 10K, and 100K resistors--the output stayed under 4V.

Researching, I found this web page, which says that the output on pin 3 will be ~1.7V less than the input on pin 8. Charles' Encyclopedia of Electronic Components Volume 2 says the same thing. I'm going to move on.