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Replace relays with solid state circuit

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hiker

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See the attached schematic for current circuit. The IN is +5v coming from computer and when present the relay is engergized. There are 8 of these which control a bi-color 7 segment LED display. The display has 2 common anodes, one for green and other for red and 7 cathodes. Seven of the cathode relays are used to switch on the display's segments by going to ground. The 8th relay is wired by N.C. leg to green anode and N.O. leg to red anode. The supply to common leg of anode relay is currently +5v but can be 5-12v.

I want to replace the mechanical relays with a solid state solution since I think it is overkill to control 5v circuit with relays. So I would like to replace with a SS device that would cause the LED cathodes to go to ground. I'm not planning to use any of the components in the current circuit but build everything new. Also the anode circuit needs needs to be a double throw type to control green and red colors. If I could control the segments by switching voltage like the relay coils are done now, I could just use the same circuit and substitute a SS device for the relay I think, but I need the inverse, ie, go to ground switching.
 

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hi hiker,
Is it one LED segment per relay, like the earlier postings.?
IIRC its about 15 to 20mA/seg
 
Yes, one relay per segment, 7 relays for 7 cathodes and 1 relay for anodes, 8 relays total.

hi,
I simple and easy way would be to use a ULN2803A, I know its an over kill in the current capacity.
The ULN2803A has 8 darlington drivers, CMOS compatable inputs.

Use 7 of the drivers for the segments and the 8th to direct drive the anode select relay.

Price is about 67pGBP each.

get the datasheet from www.datasheetarchive.com
 
Thanks, I looked at the datasheet. Sorry but I don't really have a good idea of how to wire this, especially since I need to do switching to ground for the cathodes and double throw type voltage switching for the anodes.

Can you provide a circuit schematic, please?
 
Thanks, I looked at the datasheet. Sorry but I don't really have a good idea of how to wire this, especially since I need to do switching to ground for the cathodes and double throw type voltage switching for the anodes.

Can you provide a circuit schematic, please?

hi,
Can do, are there still 8 remote display stations.?
 
Yes, 8 all identical so I should be able to just make 8 of the same.

hi,
Have a look at this circuit.

You can use either +5V or +12V for the LED's, use a segment resistor to suit the LED current you require.

The relay is a 12V.

The input fromthe computer require no resistors, direct connect.

Hope you follow OK.:)

EDIT: note pin10 is the internal clamping diodes common connection.
 

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OK, thanks, looks great.

So to made sure, the LED cathodes at points a-g will go to ground when +5v is present at pins 1-7 respectively, correct?

And to refine, can you think of a way to replace the Anode Select relay with something solid state?
 
OK, thanks, looks great.

So to made sure, the LED cathodes at points a-g will go to ground when +5v is present at pins 1-7 respectively, correct?

And to refine, can you think of a way to replace the Anode Select relay with something solid state?

hi,
It is a very simple circuit, should be easy to build, smallish size.

YES, pins 1 thru 7 control the segments.
A seg will resistor will go to about 0.6V when the corresponding input is high.

So if you used a +5V supply for the LED's and they have a Vfwd of 2V then
for 15mA you need a Rs= [5-2.6]/0.15 = 150R

I'll have a look at the semi option for the anode slection.
 
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OK, thanks, looks great.

So to made sure, the LED cathodes at points a-g will go to ground when +5v is present at pins 1-7 respectively, correct?

And to refine, can you think of a way to replace the Anode Select relay with something solid state?

hi,
This is one option for the relay replacement.

Once you decide the V for the LED's you can work out the resistor values.
 

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Also I searched Digi-Key for ZTX750 and could not find. Is there a substitute, ZTX749 or ZTX751, maybe?
 
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Also I searched Digi-Key for ZTX750 and could not find. Is there a substitute, ZTX749 or ZTX751, maybe?

hi,
ZTX749 is OK, the ZTX751 is also OK but the collector voltage rating is an overkill, so it maybe more exepensive.

I would have thought as you will be recovering the relays from the 'old' units that using a recovered relay for anode selection would be the cheaper option.
Thats the way I would go.:)
 
hi hiker,

Final circuit, tested OK.:)

Also could use ZTX450 as an alternative.

Attached also as pdf.

Whats the length of the longest cable run.?
 

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Thanks Eric.

The longest cable run is about 50ft. Is that a problem?

Also I thought of using a SPDT SSR to control the anode lines but searching for one I could only find one that has an in of 1.2v. The thought was to eliminate the mechanical relays because they can be problematic.

Are the lines at pin 8 and 11 of the ULN2803A really necessary? What power source of 5v is really supplying the power for the 7 seg display? Just want to make sure it's not the computer +5v.

There's one more option that I would like to explore. Since the display actually has 8 cathodes, one more for the decimal point which is currently unused, would it be possible to use the first method with only the ULN2803A to control the 8 cathodes and use a separate circuit for the anode select? Would it be possible to use your ZTX450 circuit separately or a simple relay?
 
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Thanks Eric.

The longest cable run is about 50ft. Is that a problem?
Not a problem, the ULN will be OK.

Also I thought of using a SPDT SSR to control the anode lines but searching for one I could only find one that has an in of 1.2v. The thought was to eliminate the mechanical relays because they can be problematic.

Are the lines at pin 8 and 11 of the ULN2803A really necessary?
Pin 8 is the input for the anode select invertor, pin 9 is the output.

What power source of 5v is really supplying the power for the 7 seg display?
The source of supply for the displays are the ZTX anode lines, how you power the inputs to the ZTX's is upto you.

Just want to make sure it's not the computer +5v.
Thats your choice, just keep the 0V lines connected together if you use two different +5V supplies.

There's one more option that I would like to explore. Since the display actually has 8 cathodes, one more for the decimal point which is currently unused, would it be possible to use the first method with only the ULN2803A to control the 8 cathodes and use a separate circuit for the anode select?
Yes that would be possible, I would add an extra transistor to do the anode inversion drive to the 2N2222's.

Would it be possible to use your ZTX450 circuit separately or a simple relay?
I dont follow what you mean.?

hi hiker,

Answered in Blue.

Why do want to control the DP on the LED.?

Is the computer driving all 8 remote LED displays, if yes, how is the computer connected to the ULN's.?
 
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I see a possible need for the DP just as another indicator or a second display that would have a fixed number alongside the current display. Just want to look at all posibilities.

Yes the computer drives all the displays via USB-virtual COMport with a Microcontroller type PIC16F876A and FT232R chip. There are 16 5v lines times 4 for 64 relays. I could add one more if needed to give an additional 16 controlled +5v lines.

Looking at your schematic, the power supply for the dispalys are the +5v just to the left of the top ZTX450, correct?
 
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I see a possible need for the DP just as another indicator or a second display that would have a fixed number alongside the current display. JuMicrocontroller type PIC16F876Ast want to look at all posibilities.

Yes the computer drives all the displays via USB-virtual COMport with a Microcontroller type PIC16F876A and FT232R chip. There are 16 5v lines times 4 for 64 relays. I could add one more if needed to give an additional 16 controlled +5v lines.

Looking at your schematic, the power supply for the dispalys are the +5v just to the left of the top ZTX450, correct?

hi,
Yes, the +5V is on the ZTX emitters.

If you have a spare computer pin, you could drive the Red and Green selects with their own pins.
You could turn on both and make it 'Amber'.?
 
Yes I could use 2 separate anode lines from the USB board but I would have to add another board for a total of 5 (max) and reprogram. It seems like a waste since the displays are always on, either green or red.

If I used a SPDT solid state relay for anodes like I mentioned that use 1.2v to control, how could I convert the 5v line from the USB board to 1.2v?
 
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