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Signal controller design issue


RobertW
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Hi,

 

I've got the below circuit. I'm driving the input from a track circuit. The system will be a 2 aspect signal driver. The circuit is currently designed for common anode signals. I'm looking for a common cathode driver. 

 

What do I need to do to convert this to common cathode. The circuit takes an active low input from the track circuit.

 

Apologies for the poor diagram, I'll try to draw it properly in AutoCAD tomorrow if required, although it's based on figure 15.2 of Roger Amos' Complete Book of Model Railway Electronics.

 

Thanks

 

Robert

17319bb4-14ac-41eb-a669-6a32a2855ae7.png

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Just an update on this, I've solved the issue. It was due to inverting the potential divider R2-R3. I've attached an updated schematic to this post, including the integral power supply and track circuit. It's designed to be fed from the 16VAC layout auxillary system.

 

I've designed this circuit for use with Train-Tech signals, which are common ground, hence the need to convert the driver to Common-Cathode. The circuit designs is based on Roger Amos' design for a signal driver and bi-directional track circuit, however, I have converted it to Common Cathode, as well as incorporating manual override. Whilst the drawing denotes the functional blocks of the system, the boards are integrated, with a track circuit, signal driver, and AC/DC converter on 1 veroboard.

 

Please let me know if you have any comments.

 

Thanks,

 

Rob

Signal_Driver_Rev_C.pdf

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45 minutes ago, RobertW said:

Please let me know if you have any comments.

 

Turning the circuit upside down and swapping the NPN for a PNP (with an extra one to invert the signal for the other LED) - pretty much what I would have suggested if you want to keep to discretes - otherwise I might have suggested a 74HC04 for the lights (you'll have enough gates for 2 signals) and a couple of resistors per signal.

 

I would use the CMOS HC series over the older LS TTL series devices as for this application they have better output current sourcing (up to 25mA) which you'll need to drive the LEDs (standard TTL has good current sinking - fine for common anode, but not much sourcing current - not so good for common cathode). 



From block detector ---------+-------------|>o------/\/\/\/------|>|----+  Red
(low to detect)              |                                          |
                             |-------------|>o-|>o--/\/\/\/------|>|----+  Green
                                                                        |
                                                                       ---GND

 

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On 05/06/2019 at 13:31, sharris said:

Turning the circuit upside down and swapping the NPN for a PNP (with an extra one to invert the signal for the other LED) - pretty much what I would have suggested if you want to keep to discretes - otherwise I might have suggested a 74HC04 for the lights (you'll have enough gates for 2 signals) and a couple of resistors per signal.

 

I would use the CMOS HC series over the older LS TTL series devices as for this application they have better output current sourcing (up to 25mA) which you'll need to drive the LEDs (standard TTL has good current sinking - fine for common anode, but not much sourcing current - not so good for common cathode). 


From block detector ---------+-------------|>o------/\/\/\/------|>|----+  Red
(low to detect)              |                                          |
                             |-------------|>o-|>o--/\/\/\/------|>|----+  Green
                                                                        |
                                                                       ---GND

 

Thanks for this input. 

 

I hadn't considered using 74XX circuits, because if I'm building a module, I generally don't have enough circuitry to use up all of the gates, and I don't like wasting the rest of the chip. On this project, I've managed to use up all the chips. I'll post the new schematic tomorrow, and I'm going to breadboard it tomorrow as well.

 

Thanks for your help.

 

Rob

Track_Circuit_Veroboard.pdf TrackCircuitSchematic.pdf

Edited by RobertW
Edited to add schematic and veroboard layout for final design
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