TomKett Posted May 3, 2015 Share Posted May 3, 2015 Hi Thanks for taking the time to read and hopefully help. I am trying to make a board to show me my point positions, the points are all operated with 'push to make' buttons. Most of my point motors are Seep PM1 so I can just take a feed directly through the point motor to power an LED, however I have a couple of Hornby Surface mount switches due to location and I therefore need to get a signal from the push to make switch rather than directly through the point motor. I am using a similar diagram to the one attached to control the point motors to pass 20v through the TIP122 transistors for a moment to switch the motor, however in this situation I need to latch the transistor open and then break the latch again when the motor switches. The supply voltage for my switches is 5v Its easiest to describe if I start at S2 S2 is pressed, this puts power onto Q1 (TIP 120 Darlington Transistor) which then pulls the 5v supply through the transitor, this doubles back to latch the transitor as on. Power then passes directly through Q2 ( This is the opposite of a TIP120 and breaks the connection if power is supplied through the pin1, not sure what transistor I need?) To Light the bicolour LED D1 and the opposite colour on D2 When the point is then switched by pressing S1, this puts power through Q2 which cuts the latch, power is also applied to Q4 which latches the second Tip120 on and therefore the LED's have now reveresed. Now when S1 is pressed again, Q3 is now used to break the latch and the original latch @ Q1 is now applied. Reversing the LED's again. All other Diodes are IN4001. Hope this makes sense. My questions are. Does the diagram make sense? What transistor should I use for Q2 & Q4? Thanks again for taking the time to read. https://www.dropbox.com/s/r4z58s2uxe8r7zi/Signal%20Latching.jpg?dl=0 regards Tom Link to post Share on other sites More sharing options...
smokebox Posted May 3, 2015 Share Posted May 3, 2015 Have you considered using latching relays instead? Link to post Share on other sites More sharing options...
TomKett Posted May 3, 2015 Author Share Posted May 3, 2015 Thanks I may see if I can do that, was trying to use transistors to keep the cost down but only have a couple of these point motors Link to post Share on other sites More sharing options...
Gordon H Posted May 4, 2015 Share Posted May 4, 2015 Most of my point motors are Seep PM1 so I can just take a feed directly through the point motor to power an LED, however I have a couple of Hornby Surface mount switches due to location and I therefore need to get a signal from the push to make switch rather than directly through the point motor. I am using a similar diagram to the one attached to control the point motors to pass 20v through the TIP122 transistors for a moment to switch the motor, however in this situation I need to latch the transistor open and then break the latch again when the motor switches. The supply voltage for my switches is 5v Its easiest to describe if I start at S2 S2 is pressed, this puts power onto Q1 (TIP 120 Darlington Transistor) which then pulls the 5v supply through the transitor, this doubles back to latch the transitor as on. Power then passes directly through Q2 ( This is the opposite of a TIP120 and breaks the connection if power is supplied through the pin1, not sure what transistor I need?) To Light the bicolour LED D1 and the opposite colour on D2 When the point is then switched by pressing S1, this puts power through Q2 which cuts the latch, power is also applied to Q4 which latches the second Tip120 on and therefore the LED's have now reveresed. Now when S1 is pressed again, Q3 is now used to break the latch and the original latch @ Q1 is now applied. Reversing the LED's again. All other Diodes are IN4001. Hope this makes sense. My questions are. Does the diagram make sense? What transistor should I use for Q2 & Q4? Unfortunately, your diagram won't work as shown. Your latching concept is flawed because the voltage that appears at Q1 emitter when you press S2 will be too low to provide any bias current via the diode to keep Q1 switched on. Also, the transistor type shown for Q2 & Q3 would be incorrect. These would also be need to be NPN to stand any chance of working.. Not sure where in this diagram your point motor coils are located either... Link to post Share on other sites More sharing options...
Baby Deltic Posted May 4, 2015 Share Posted May 4, 2015 I can't work out why you'd want to use Darlington's for such a circuit. As Gordon points out, I can't see how the circuit could function. If you don't want to use latching relays, you could use bistable multivibrator a which require only two transistors, a 555 in bistable mode, or even a pair of thyristors. Link to post Share on other sites More sharing options...
Baby Deltic Posted May 4, 2015 Share Posted May 4, 2015 Im not sure, but I would think that a transistor cannot be latched if there is no current path from the emittor to ground. I don't think the diode loop will work. If Q2 is not in conduction, there can be no current flow through Q1. Q2 cannot be triggered until S2 is pressed (if it was NPN). PNP bipolar transistors need a negative base voltage, or ground potential to switch on, unlike NPN's which have a positive switched base region. Also, PNP's have the emittor towards the positive rails and the collector negative, so they look back to front. I take it the diodes at the bottom are flywheel diodes where the coils would be? These need to be reverse biased or you will get a loud bang. Link to post Share on other sites More sharing options...
Baby Deltic Posted May 4, 2015 Share Posted May 4, 2015 Ive done a quick sketch on the back of an envelope to show a circuit using a TX2SA-L2-12V twin coil latching relay and a couple of discrete components: Sorry its a bit scruffy and I haven't shown the relay contacts as N/O - N/C changeover which they are. Basically one switch activates coil one, which flips the relay into one state, changing the double pole changeover contacts to their opposite state, and the other switch causes the other coil to be energised, causing the relay to flop back into its original state. The two separate pairs of changeover contacts can be used for switching the LED's on one side, and switching the point frog polarity on the other, thus ensuring good continuity even if the point blades make poor contact. The two IN4001 diodes act as flywheels to collapse any reverse currents when the coil de-energises, thus protecting S1 and S2 from high current arcing. I have calculated the resistors as 470R, which should give 3V at 20mA across each LED. A link to the similar 2Q2SA-L2-12V relay. They are shown as surface mount, but you can bend the pins down and they are long enough to go through veroboard or similar: http://www.ukcomputersandsoftware.co.uk/product-2A-12VDC-DPDT-Surface-Mount-2-Coil-Latching-Signal-Relay-TX-Series-Relays-amp-Solenoids.html Link to post Share on other sites More sharing options...
Junctionmad Posted May 5, 2015 Share Posted May 5, 2015 If you want a simple set reset latch type of operation Here's a simple circuit unit you can adapt http://www.bowdenshobbycircuits.info/page9.htm Link to post Share on other sites More sharing options...
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