Intercom control circuit

Thread Starter

AnalogKid

Joined Aug 1, 2013
8,464
NOTE: C3 connects to +5 V, not +12 V.

Please review *this* circuit. It is a controller for an intercom station, and uses a momentary SPST switch to function as both momentary and alternate-action. Yes, it can be replaced with PIC. That is not today's topic.

IntercomControl-2-c.gif

The story:

Back in another life I worked nights at a PBS station. Separate from the production intercoms in the studios, the building had an intercom that linked the Videotape room with the two studio control rooms, TV master control room, FM master control, and the production offices. Each intercom panel had a mic, speaker, and a row of 12 4PDT leaf switches that were momentary down and latching up, with one pole (microphone mute) being closed in the center position and open in both the up and down positions.

The switch array was a continual maintenance problem, so I worked up an all-electronic replacement. The idea was that a computer keyboard switch designed for low bounce and millions of operations would be more reliable. The problem was how to have a SPST momentary switch emulate both the momentary and latching functions of the old switch.

The solution was timing. Pressing the button longer than 0.5 s was interpreted as a momentary action: talk while pressing, release when done. Pressing for shorter than 0.5 s was interpreted as alternate-action: tap to latch on, talk hands-free, tap to release when done. The circuit controlled analog switches that manipulated the audio and control signals such that nothing else in the system had to change.

The ***problem*** was that I never could get the circuit down to one chip type per icom channel. The best I could do was 1/2 of a D flipflop plus one 2-input gate. This was in the early 70's when chips were relatively expensive, and we had a budget of almost zero dollars, so parts optimization was a real thing. Four channels took three chips, and that extra gate just pissed me off.

40+ years later, I think I finally got rid of the gate. This is preparation for building a house intercom based on a Howland current source party-line system. This is a mixed-signal circuit, and I don't know how to simulate it in LT Spice.

R1 and C1 debounce SW1 and turn U1A into a toggle ff. Tap SW1 for less than 0.5 s and the ff latches a 1; tap it again and it latches a 0. This is the alternate-action function, and is pretty straightforward.

For the momentary function, the fun happens at the Reset input. In the rest state, C2 is charged up to 5 V. As SW1 bounces, the R input bounces between 5 V and 10 V. I had to add R4 to keep U1's input protection from trying to clamp C2. During the bouncing, C2 charges and discharges a little bit on each half-cycle. After the bouncing has stopped C2 steadily discharges through R3, but not enough that the R input sees a logic 0 IF SW1 is released in under 0.5 s.

If SW1 is held for longer than 0.5 s, C2 discharges through R3, enough that when SW2 is released. R2 pulls C2 and the R input down low enough to be a logic zero. This resets the ff to the rest state.

Tap > on, tap > off. Hold > on, release > off. None of the inputs are hysteretic. Vth = 0.5 Vcc, and time constants = 0.7 x R x C.

ak
 
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Thread Starter

AnalogKid

Joined Aug 1, 2013
8,464
Hey AK, I tried. Not sure if your design or the chip in Spice:
Third optiion - operator headspacing adjustment.

I show Vcc = 12 V. It is 5 V, same as the other three connections. The design started out with a 4013 and Vcc = 12 V. I had to change to the 7474 to get the correct Reset input logic polarity, and missed one of the symbols. Please set V3 = 5 V and retest.

ops.

Suggested test waveforms:

1. 0.2 s high, 1.8 s low, repeat. (alternate-action test)

2. 2.0 s high, 2.0 s low, repeat. (momentary test)

Thanks for your help.

ak
 
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Wolframore

Joined Jan 21, 2019
1,748
another flip flop... this one also getting hung up after first - gotta get ready for my finals tonight :) try again another time.
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Danko

Joined Nov 22, 2017
928
Nice I have another design idea which also works. Just need the not Q on data. I’ll work it out. @Danko could you share the flip flop model?
Here it is.
Added:
Seems voltage source can not be used as switch, because it connects capacitor to ground when low level.
Added:
With CD4013B circuit works too, but needs to swap +5V and ground (in squares), and inverting clock signal.:(
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Wolframore

Joined Jan 21, 2019
1,748
Isn't the 4011 a quad nand gate chip... I plan on going on to logic classes next fall so I would love to get my logic chips in order. ohhh i misunderstood... gotit.. .you need the nand gate to switch the logic. I've been struggling with the 74hc.lib and the 74hct one as well. I would rather work with the hc as these are what I usually go for.
 

Wolframore

Joined Jan 21, 2019
1,748
For logic I may just use Proteus... works well and has some nice features:

I couldn't get logic oscillators to work in either simulators. It seems that our logic chips are too perfect and no noise to start them.

1589392302201.png 1589392319201.png
 

Wolframore

Joined Jan 21, 2019
1,748
Of course schmitt trigger oscillators are different, they work fine in any sim but this is due to integration:

1589393226261.png

Sorry AK, I seem to have gotten a little distracted trying to get this stuff to work
 
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