Make BJT FF remember its state?

ian field

Joined Oct 27, 2012
6,536
Here's one for the transistor gurus!

I've got a BJT flip flop circuit I've been playing with....2 NPNs, and 2 PNPs. The PNPs are driving 2 LEDs. Nothing fancy.

This is so my father in law (the railroad buff!) can switch tracks using a DPDT toggle (center off), and have an indicator light for which way the tracks are now set.

Ok, doing this is easy. BUT - REMEMBERING which light should start "ON' - that is not so easy! There is no electrical connection between the switch machine or tracks and this circuit. I'm thinking I may HAVE to provide one or this may prove impossible.

Any suggestions on how to force one LED to be high at power-up? :confused:

Thanks!
To remember the bistable state since the last power down - a carpenter relay is the simplest.

Alternatively - a CMOS bistable with a lithium coin cell backup battery.
 

inwo

Joined Nov 7, 2013
2,419
A couple more questions if you don't mind.

Do you know at what voltage the switch machine operates?

If you hold the momentary contact closed, will the "track switch" go open circuit when switching function is complete? Either by a relay contact or end switch.

If not, would it be easy to do?

Using an end switch on each position, the track that is selected, could easily be monitored and displayed at the toggle switch end. With no additional wiring.
 

Thread Starter

Mike33

Joined Feb 4, 2005
349
Ian - what's a carpenter relay? A latching relay? Sorry, I couldn't find a reference to carpenter relay via search engine.


Inwo: the "Tortoise" operates at 13.8 VDC. Neat little gadget...I put a 470u cap in parallel with the terminals, and that makes it go its complete travel with just a quick burp on the momentary switch. If it WERE to be held in a stall position, or switch held on, it will not hurt the Tortiose (just would waste a little power). It does not go open circuit as wired now - again, the cap does the trick, and spends itself quickly once the arm is over where it belongs.

A solution with little wiring sounds great, because yes, it's expensive, and yes, a LOT of work! Some of these 10 Tortoises are 20 feet away. A latching relay is looking more acceptable all the time, ha ha...
 

ian field

Joined Oct 27, 2012
6,536
Ian - what's a carpenter relay? A latching relay? Sorry, I couldn't find a reference to carpenter relay via search engine.


Inwo: the "Tortoise" operates at 13.8 VDC. Neat little gadget...I put a 470u cap in parallel with the terminals, and that makes it go its complete travel with just a quick burp on the momentary switch. If it WERE to be held in a stall position, or switch held on, it will not hurt the Tortiose (just would waste a little power). It does not go open circuit as wired now - again, the cap does the trick, and spends itself quickly once the arm is over where it belongs.

A solution with little wiring sounds great, because yes, it's expensive, and yes, a LOT of work! Some of these 10 Tortoises are 20 feet away. A latching relay is looking more acceptable all the time, ha ha...
Its a relay that has a magnetically polarised armature - that does 2 things, first - the armature sticks to whichever pole its directed to, secondly the pole can either attract or repel the armature depending on the direction of current through the coil.

Some types have a single coil - you have to reverse the current flow to reverse the latched state, some types have 2 coils.

If google doesn't recognise carpenter relay - try bistable relay.
 

Thread Starter

Mike33

Joined Feb 4, 2005
349
Thanks. This does look like a case where investing in a relay will solve many other problems. The flip flop is a 'cool transistor idea', but sometimes you have to drop the cool idea for the practical one! Spend $2 now, save $10 in wire, etc.

I found bistable relays...
 

inwo

Joined Nov 7, 2013
2,419
Thanks. This does look like a case where investing in a relay will solve many other problems. The flip flop is a 'cool transistor idea', but sometimes you have to drop the cool idea for the practical one! Spend $2 now, save $10 in wire, etc.

I found bistable relays...
Using the same idea of moving an armature one way or the other, you may be able to modify the relay to move a small flag, rather than led.

That would be a cool steam punk indicator for a railroad.
 

John P

Joined Oct 14, 2008
2,025
If you're using the Tortoise machines, then you have a kind of built-in "memory" available there, at least in theory. (For anyone who doesn't know, this type of unit is operate by a very low-current motor, not a solenoid, and it's intended to move to the end of its stroke and stay there constantly stalled until you reverse the motor.)

Let's say the turnout's settings are A and B. You would start by powering the motor to send it to position A. If current is above a threshold, you were already in position A (motor is stalled). If it's less than the threshold, the motor must be moving, so its initial position was B. In that case, the circuit immediately reverses and drives the motor back to B, and so the information on what the turnout setting was has been extracted, and the right LED can be lit. Now, I will not deny that if anyone here asked how to make such a circuit, I'd say "Get a microcontroller". But maybe there's a smart person here who can figure out how to do it in some simple inexpensive way without a processor.

If anyone's interested, there is a Yahoo "Model Railroad Electronics" group:
https://groups.yahoo.com/neo/groups/mrrelectronics/conversations/messages
 
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