GPO Lamp control (LK204-25)

JohnCox · Post by **JohnCox** » Fri Dec 06, 2002 10:53 am

My project involves multiple LK204-25 LCD displays, each controlling several small lamps. The lamps are GE 555 bulbs, 6 volt, 0.25 amp. I have a separate power supply for the lamps, but need a switching circuit that will work with the GPOs. The project will require about 30 lamps and thus 30 GPOs, so that each lamp can be controlled independently. All of the lamps will be in fairly close proximity to each other, so a single board could be used if needed.

Could anybody please describe a cost effective solution for implementing these circuits? Are there any off-the-shelf boards that can be purchased or would it be cheaper to build one? Any specific design details or component sources would be appreciated, as this would be my first attempt at building a board.

Thanks for any suggestions!

Post by **linear** » Fri Dec 06, 2002 12:50 pm

Have a look at this. The transistor speced in that will get you up to a 3A load current. At under a dollar each for the transistors, I would call that cost-effective.

JohnCox · Post by **JohnCox** » Fri Dec 06, 2002 5:18 pm

thanks linear, that's just the info that i was looking for!

JohnCox · Post by **JohnCox** » Fri Dec 06, 2002 7:54 pm

After studying the BayBus circuit, I am a little confused about one thing. When the GPO is OFF (i.e. the negative pin is NOT grounded), where does the base (B) voltage come from to switch on the transistor? Thanks for the help, and please excuse my electronic ignorance.

In short the circuit works as follows, when the GPO is 'on' its -VE pin is earthed and no current can flow into the Base (B) of the transistor, so the transistor is off and nothing happens. When the GPO switched off, a small current is drawn from the GPO's -VE pin which is sufficient to fully switch the transistor on. This allows a current to flow from the Collector (C) to the Emitter (E) and hence the high power output switches on.

BayBus Circuit:

GPO Circuit:

Post by **linear** » Fri Dec 06, 2002 9:44 pm

What it's saying is that the GPOs are active low. If you just slap an LED across it, the LED lights when the -ve pin transitions from high to low. The +ve pin is nailed up at +5V permanently.

>where does the base (B) voltage come from to switch on the transistor?

So if the GPO is OFF, it means the -ve pin is HIGH, so the base-emitter junction is forward biased, and the transistor conducts, making current flow to the load. So the transistor is inverting the sense of the GPO here. Conversely, with the GPO ON, the _ve pin is LOW, so the BE junction is not forward biased, no current flows, making the load get turned OFF.

The base current he mentions is on the order of 1 mA, so you're a long way from the rating of your GPOs, this is a stress-free way to go after it (as opposed to relays--ugh).

JohnCox · Post by **JohnCox** » Sat Dec 07, 2002 9:24 am

OK, I'm going to order the components this week to build a few of these circuits for testing with my project. Are the TIP31A transistor and the 4.7k resistor adequate for a 6 volt , 0.25 amp load? The circuit designer claims that he can handle 3 amps at 12 volts. Any problem with using a lower voltage on the load side (5-6volts)?

Thanks o' plenty for all of the wonderful help!!

Post by **linear** » Sat Dec 07, 2002 3:33 pm

No problem at all. Load voltage at 6V poses no problems. And any transistor that can handle 250 mA collector current will do the trick (in case you feel like going bargain hunting).

JohnCox · Post by **JohnCox** » Sun Jan 12, 2003 8:20 pm

Well, I tried building the circuit but it didn't work.

Here's how I wired it up on the breadboard:

I know that the GPO output is working because a used a LED to test it. My transistor is a TIP31C, instead of a TIP31A. Would that make a difference?

The resistor is 1/4 watt, 4.7K ohm. The bulb is 6 volt, 0.25 amp and burns nicely when connected straight to the power supply.

I am using separate power supplies for the lamp and the LCD. The lamp is powered by a 5v wall transformer. The LCD is powered by the PC's power supply.

I not really good at electronics and was trying to follow the diagram that was used in the baybus circuit. I would really appreciate any help!!

Any ideas? Thanks!!

Stig · Post by **Stig** » Mon Jan 13, 2003 5:11 pm

If all these drawings are correct, this circuit will not work (as you have found out...). The GPO in the drawing a couple of posts up, is basically an open-collector output, i.e. you need something to pull it up when it is switched off. Shorting the GPO, would do this and because of the limiting resistor, limit the current to 21mA. Otherwise you could connect the resistor to +5V and base, and another connection from base to GPO. This would be the same, just would decrease the current going in to the transistor (and also through the GPO).

Just looked at the datasheet, and if my long forgotten understanding of transistor characteristics is correct, you need a resistor of about 1kOhm. But it would be better if Henry or Aniso would comment on this....

Also if you needed to send a lot of current through the transistor, you would also need a driver for it, since the hfe is only 20-50.

Stig

Post by **linear** » Mon Jan 13, 2003 8:22 pm

I think the circuit is fine, but your grounds are referenced to different points.

the reason there's just one wire into the base from the GPO is because the GPOs are sharing the display's ground connection. I bet if you tied your wall wart's ground to the PSU ground what you have will work (I'll trust you on the pinout, I don't have the datasheet handy to check).

What turns the transistor on is a forward-biased base-emitter junction. IF you wanna troubleshoot just the transitor switching portion, try connecting +5V to the other end of the base resistor--current should flow in the collector (and the load).

JohnCox · Post by **JohnCox** » Mon Jan 13, 2003 10:26 pm

Thanks Stig & linear.

Here's what I did...

With the GPOs unplugged, I applied 5v to point A to test the transistor thingy. (If your drawing in the earlier post doesn't have labels, please hit your refresh button.) The lamp came on, but it was very dim. So I placed another 4.7K resisitor in parallel to the first to allow more base current (if I'm saying that right). The light burned brightly, and about the same as it did when I grounded point C.

Ok, so the transistor switching works, I reckon.

So how do I wire it to the GPO? If am reading you right, linear, you say tie the grounds together by shorting points E&G. And Stig says to short both GPO pins to point A. (Or, put 5v at A and connect GPO[-] to B instead). Please let me know if I'm understanding you both correctly.

Ok, so which is better? :-D How about my resistor/transistor/load sizing? The load is 0.25amp@5vdc. I figure that two parallel 4.7k resistors equate to a 2.35k. Is this sufficient or should I use a 1k?

Thanks guys for all of your help!! I've got to make 30 of these.

anlamotte · Post by **anlamotte** » Fri Jan 17, 2003 2:56 am

Hi...!

The GPO is an open-collector output, therefore it's easier to
use a PNP transistor insted of a NPN. check this circuit:

this uses PNP transistors to offload the GPO interface, the
first one doesn't need to be a big one.. a cheap BC556 will
do. Transistor #2 needs to be capable to handle to total
current of the lamp.

It's very imported to have the same ground potential. Make
sure to ground's are connected together. The input to the
first transistor goes to GPO V- and the other line to ground.
GPO V+ is not used in this circuit.

Stig · Post by **Stig** » Fri Jan 17, 2003 4:22 am

This circuit could work, IF you swap around emitter and collector from what I could tell (have no means to test it here...). For a PNP-transistor to open, base has to have a lower potensial than the emitter, which it will not get in this circuit.

JohnCox: If you haven't gotten any further, you could try Linear's suggestion (you have to connect the grounds together anyway) and see if it works. If it does, GOOD! If it doesn't, you could try my suggestion with a resistor to +5V to base and a connection between base and GPO (possibly with a small resistor in between to start with) and see if that works.

Edit: I'll get home late next week, so I can see if I can get hold of a TIP31 and test this at home.

Stig

JohnCox · Post by **JohnCox** » Mon Aug 30, 2004 7:55 am

Sorry to revive this old topic, but my project has been put on hold for a while. Recently, there has been a renewed interest around here, so I will be picking up the pieces very soon.

Let me preface this by saying that I very much appreciate everyone's help. My expertise is in software engineering, not EE. So, please forgive any misguidings.

I never tried tying the grounds together, because I wasn't sure if it was safe. I ordered the wide voltage option (7-15v) on my LK204-25 and its powered by my PCs power supply. For the lamp, I'm using a 5v wall transformer to power my breadboard.

In the production model, I will opt for the 5v voltage option on the LK204s and use a common powersupply that can handle the 10 LCDs & 30 lamps.

Would anyone be interested in building these circuits for a fee?

JohnCox · Post by **JohnCox** » Mon Aug 30, 2004 11:45 am

Here's what I think I would need. The project would require 10 of these boards.

GPO Lamp control (LK204-25)

GPO Lamp control (LK204-25)

Use a PNP transisitor....!