Radio Push Button - The Journey

Thread Starter

iimagine

Joined Dec 20, 2010
489
I'm creating this post for future references. A while ago I got bored and decided to do a design exercise on this, back then I called it 'Push Button Selector' because I did not know of the term 'Radio Push Button', and hence I could not do no research on it. I had to start from scratch - https://forum.allaboutcircuits.com/threads/push-button-selector-switch-no-ics.161832/ - Months later, reappears in - https://forum.allaboutcircuits.com/threads/sr-latch-within-an-sr-latch.168202/ - This time, Mr Crutshow has simplified it significantly and I think it is as good and simple as it could get, here is the circuit, slightly adjusted to ensure the saturation of all NPNs:
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crutschow

Joined Mar 14, 2008
27,930
Did you do a simulation?
My LTspice simulation does not work with those resistor values:
The LED does not shut off when the next PB operates.

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vu2nan

Joined Sep 11, 2014
188
Here's my version, using a diode matrix, push button switches and magnetic latch relays.

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NO' contacts of the relays K1, K2 & K3 are to be used to switch signals,

Relays with gold plated contacts are a must if low level signals are to be switched.

Freewheeling diodes, for the relay coils, and LED indicators are not shown.

-Nandu.
 
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crutschow

Joined Mar 14, 2008
27,930
Yes, it works fine on Falstad circuit simulator. I wonder why it is not in LTSpice..
Could be the generic transistors used in Falstad and a simpler simulation program.
I don't know what simulator engine they use in Falstad, but Spice is generally the most accurate of the analog simulators around. Most commercial analog simulators use some variation of Spice.
I've seldom found a significant deviation from the Spice emulation and the real circuit so, if the simulation fails, I'm fairly confident that the real circuit will also have problems or be marginal.
Usually if Spice is off, it's because the particular device model is not accurate.
I suggest you learn to use LTspice (it's probably the best of the free Spice programs) if you are serious about simulating your analog designs.
It has a somewhat steep learning curve, but the many included example circuits, and the online tutorials are a big help.
 

Thread Starter

iimagine

Joined Dec 20, 2010
489
I suggest you learn to use LTspice
I do use it sometime (I know a little) to double check my circuits, I find it tedious thats why I mostly use Falstad first. You were right, it didnt work. Not enough voltage changes at the common emitter resistor. To fix this, I use the voltage drop at the LED to bias the NPN instead.
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crutschow

Joined Mar 14, 2008
27,930
Below is probably my favorite Radio PB circuit as it requires only 1/2 a NAND gate package (1/4 package if CD4044 is used), 2 resistors and 1 diode for each added pushbutton, and it draws only leakage current when in the static state so its setting can be readily backed up with a small coin cell if needed.

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eetech00

Joined Jun 8, 2013
2,600
Hello

Here's a four button version. Its not very robust unless the momentary PBs are interlocked.
But posting anyway cause I'm bored too.:D
Its a busy circuit that would be better served using a digital chip(s). But posting it anyway.
The circuit is basically four BJT latches with a reset line. I haven't bench tested this but works in simulation.
see below.

eT

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MrChips

Joined Oct 2, 2009
24,424
You can connect an MC14532 8-input priority encoder to an MC14028 3-to-8 decoder.

For a single chip solution you can use an MCU.
 

crutschow

Joined Mar 14, 2008
27,930
Here's an example of the first circuit described by MrChips except it shows a 4514 16-bit decoder.
It works for up to 8 inputs as shown but can be increased to 16 inputs with a second 4532.

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crutschow

Joined Mar 14, 2008
27,930
but also needs switch debouncing?
Debouncing is only needed if more than one pulse will affect the state of a circuit (such as the clock for a counter).
In this case, you can push a particular button multiple times, the output just stays in the same state (the state for that push-button) so switch bounce is not a problem.
The same is true for the set and reset inputs to a latch or flip-flop.
 

eetech00

Joined Jun 8, 2013
2,600
Debouncing is only needed if more than one pulse will affect the state of a circuit (such as the clock for a counter).
In this case, you can push a particular button multiple times, the output just stays in the same state (the state for that push-button) so switch bounce is not a problem.
The same is true for the set and reset inputs to a latch or flip-flop.
I guess it could cause a problem if multiple buttons were pressed at the same time?

eT
 

Thread Starter

iimagine

Joined Dec 20, 2010
489
I think the cheapest option so far is the 2 transistors per button solution . If we were to replace the PNP BJTs to PMOS and re-bias it could drive anything. IC solutions are great, but still need MOSFETs to drive anything other then LEDs.
 
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