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 a PIC. That is not today's topic.
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
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 a PIC. That is not today's topic.
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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