Help required to obtain alternate forward/reverse pulse from single push-button

crutschow

Joined Mar 14, 2008
34,692
It has not been stated what would happen if the device received two pulses of the same direction in a row.
I would not think that's a problem since I doubt the original switch has an interlock to prevent to pulsing the switch in the direction it already is.
 

AnalogKid

Joined Aug 1, 2013
11,145
I get that the device would not move, since it already is at the end of that direction of travel. But would it overheat? strip a gear? etc?

ak
 

djsfantasi

Joined Apr 11, 2010
9,183
The following would be to use to replace a spring-to-centre toggle switch (which I don't like) with a single push-button.

I would like (if at all possible) to use a single push-button, which when pressed will send a short forward(?) pulse, and when pressed again will send a short reverse(?) pulse.

The final unit downstream requires pulses, NOT constant signal.

Could this be either by the push-button be momentary/non-latching and having 3 (or more?) pins for common(?), forward(?), and reverse(?), so that alternate presses would send forward/reverse pulses (if such a push-button is available, which I doubt);

OR,

it having only 2 pins, which would send a pulse to some an intermediate component/circuit, which in turn would send forward/reverse pulses downstream on alternate presses of the push-button.

Is such an idea possible, and if so, what other components would I require.

Thanks.
Who’s the manufacturer of the turnout motor? Is it a twin coil machine or a slow-motion machine (I.e. Tortoise?)
 

djsfantasi

Joined Apr 11, 2010
9,183
I get that the device would not move, since it already is at the end of that direction of travel. But would it overheat? strip a gear? etc?

ak
It’s likely a twin coil turnout motor, from the TS description. Simply two solenoids. Multiple sequential pulses to one coil do nothing. And since it’s just a solenoid, there is nothing to strip. However, a long pulse could burn out the solenoid.

There are other types, such as a slow motion turnout motor, but they are made with a geared motor and have a small stall current. I don’t think they are of this type because they’re not typically operated with a pulse.

I’d use a CMOS T flip flop (with Q and ~Q outputs. Like a CD4013) driving N channel MOSFETs and a pull up resistor to each input. Simple and effective.
 

Thread Starter

Deltech

Joined Feb 18, 2021
7
Deltech, Do you have the internal schematic of the points motor ? Now that you mention the two indicator LED's the output from these might give us some alternative ideas. The signals from the LED's may remove the need for a flipflop.

Les.
Thank you for all the replies.
I do not have the same internal schematic, but have (hopefully) below attached the wiring diagram diagram that comes with the solenoid points (turnout) motor.

As you can see, the manufacturer SUGGESTS using a SINGLE centre-off switch or push-buttons (x 2) or stud contacts to send pulse to switch A or switch B.

NOTE:~ I use the word PULSE but it is actually a constant signal for as long as the switch or button is pressed.

Another option (not in list) is a “passing contact switch” designed to look like a miniature lever in a trackside signal box (photo hopefully below).

Terminal “F” outputs a +ve or -ve (in this case 12v) to the frog (that’s the vee-shaped rails in the centre of the turnout that move to determine the route).

Red and green LEDs or a bi-colour LED (not shown) are fitted on controller route indication panel and also connected to “F”.

”F” is also used to feed red/green signals (not shown).

It is vital that power is not maintained to “A” or “B” after the solenoid operates or the solenoid will burn out.

This is why most people use 2 x push-buttons OR centre-sprung on-off-on toggle switch for each turnout to operate alternative routes, because the solenoid is only energised briefly whilst the button or switch is operated.

But I do not like the controller panel like that because it gets too “claustrophobic“ and I would like to streamline it with only 1 x push-button for each turnout.


I hope this information will assist you in understanding my challenge.

Small pic “passing contact switch” IGNORE.

Large pic connections on turnout solenoid motor.

28E7CEAC-873A-42D3-97B9-D740B7DCEDF8.jpegDBB6C15A-35EC-406C-93A4-2836C7C01AF3.jpeg
 
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Thread Starter

Deltech

Joined Feb 18, 2021
7
I hope this information will assist you in understanding my challenge, which is to build a circuit using a SINGLE momentary switch as the input, but each time it is pressed it gives alternate +ve or -ve PULSE across terminals “A” and “B” on the turnout solenoid.
 

djsfantasi

Joined Apr 11, 2010
9,183
Another approach is to combine one pushbutton and a SPDT or DPDT toggle switch.

Set the toggle to the desired position and press the pushbutton. The pushbutton is wired in series with the common on the toggle. The other two terminals on the toggle are wired to each coil.
 

Thread Starter

Deltech

Joined Feb 18, 2021
7
Another approach is to combine one pushbutton and a SPDT or DPDT toggle switch.

Set the toggle to the desired position and press the pushbutton. The pushbutton is wired in series with the common on the toggle. The other two terminals on the toggle are wired to each coil.
That is not the solution because it still leaves me with 2 buttons/switches on my control panel when my challenge is to REDUCE to one.

And, it INCREASES my actions from one (operate original toggle switch) to TWO (operate toggle switch AND button).
 

djsfantasi

Joined Apr 11, 2010
9,183
That is not the solution because it still leaves me with 2 buttons/switches on my control panel when my challenge is to REDUCE to one

And, it INCREASES my actions from one (operate original toggle switch) to TWO (operate toggle switch AND button).
Fair enough. I’d thought to suggest it because it is the prototypical solution used on 1:1 railroads on their CTC panels. Many model railroaders strive for prototypical realism.

I almost gave you the solution you desire in post 27. I missed adding a 555 one-shot driving a P-channel power MOSFET to pulse the common connection of the two solenoids. Plus, it needs a debounce circuit for the pushbutton and snubber diodes on the coils.

If you can code, I suggest an 8 pin MCU, such as an ATTiny, instead of the debounce circuit, one-shot and T flip flop.
 
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eetech00

Joined Jun 8, 2013
4,006
Terminal “F” outputs a +ve or -ve (in this case 12v) to the frog (that’s the vee-shaped rails in the centre of the turnout that move to determine the route).

Red and green LEDs or a bi-colour LED (not shown) are fitted on controller route indication panel and also connected to “F”.

”F” is also used to feed red/green signals (not shown).
So..."F" outputs either 12v or 0v depending on the position of the turnout?
If so, can you show the existing circuit attached to "F". We might be able to use this in the new control circuit.
 
@eetech00 Doesn't the CD14538B require a decent edge? i.e. Is a schmidt trigger necessary?

The monostable makes it more reliable, but in reality it's not really necessary. You would hold it until you saw a state change or, in essence, hold for the same time as the spring loaded switch.

the timerblox https://www.analog.com/en/technical...ckly-and-reliably-solve-timing-problems.html# series of devices from Analog Devices have always intriged me but they are 5V only.

This https://www.analog.com/media/en/tec...heets/LTC6993-6993-1-6993-2-6993-3-6993-4.pdf has a solenoid driver at the bottom. Instead of a transitor, a logic FET could be used.

There are a bunch of contactor debouncer IC's that can be used. https://www.edn.com/ultimate-contact-debouncer/
These would reduce circuit complexity.

ELM makes a number of interesting products for debouncing. https://www.elmelectronics.com/products/ics/bench/#switches

Don;t forget wetting current issues. The contacts might not be rated for relaiable low current signals. 1-10mA is a good rule of thumb for the current needed to switch to keep contacts clean.
 

eetech00

Joined Jun 8, 2013
4,006
@eetech00 Doesn't the CD14538B require a decent edge? i.e. Is a schmidt trigger necessary?
The CD14538 Dual Timer has schmitt trigger inputs.
I used monostable so I would'nt have to deal with the state of a Flip flop.:)

I've update the circuit based on new information.

The Qbar outputs will provide clean edges to the mosfets.
I've shown simple RC debouncer (and pull down) for the Frog switch feed but it may not be needed.
The Frog switch holds the appropriate timer in reset.
Differentiator at PB input eliminates the effect of holding the PB down too long.

1613877144687.png
 
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crutschow

Joined Mar 14, 2008
34,692
Okay, after a better understanding of how everything works, below is the LTspice simulation of a circuit that is reasonable simple, should do what you want, and requires no IC's.
It uses the Frog (F) signal and some diodes to allow a pulse only to the opposite (unlatched) solenoid.
The pulse width is determined by the value of C4-R8 and C5-R11.

1613972638282.png
 
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LesJones

Joined Jan 8, 2017
4,212
Deltech, There are a few things I don't understand. In post #3 you talk about the system being powered by 19 to 24 volts AC or DC. In post #28 it shows a 12 volts DC supply. Are both power rails used ? What does "CDU" stand for ?
I have done some Googeling on the "seep PM1" and managed to find some pictures of it. Can you tell me if these assumptions are correct ? One solenoid is connected between A and C. The other solenoid is connected between B and C. D, E, F are a set of changeover contacts with F being the common. When the points are in one position F connects to D. In the other position F connects to E. Can you confirm that there is no electrical connection between these contacts and the solenoids ?
Looking at crutschow's schematic in post #37 I think he has come to the same conclusions. I think that circuit is about as simple it can be made. I don't think it would be worth using a microcontroller to replace the logic part of the circuit on the left hand side as the same components would be required for the solenoid drivers.
Do you use the connections D, E, F for any purpose at the moment that would prevent them from being used in crutschow's circuit.

Les.
 

djsfantasi

Joined Apr 11, 2010
9,183
@LesJones CDU = Capacative Discharge Unit. Old-style model railroad turnout machines use two solenoids to move the track points. A CDU supplied enough current to activate a solenoid without burning them out.
 

LesJones

Joined Jan 8, 2017
4,212
Thanks for the explanation. That seem like a good way to provide a large current pulse without providing a steady current that could burn out the solenoid. It probably also saved the cost of a higher rated transformer.

Les.
 
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