18x standard board darlington help

J

juve021

Member
Hi all, I'm trying to use a standard 18x project board with onboard UL2803 darlington npn array to light some LEDs.


The LEDs are packaged in a button circuit board and have surface mount resistors so I'm not sure their values. I know that if 12v are applied they light up (with no heat issues). The three resistors have a common ground.

My question is, how would I wire the outputs of the UL2803 to control a common ground. I know usually I would connect 12v to the LED (via proper resistor) and the ground wire of the LED to the UL2803 output but I can't do this in this case because of the common ground between the three LEDs.

Help please!
 
M

MartinM57

Moderator
I'm afraid you can't :(

You would have to use a "high side" equivalent of the 2803 to source current to the "top" of the LEDs (e.g.UDN2981), but that wouldn't fit on the project board (well it will fit, but it won't work - smoke could escape big time) :(

PS - you're PICAXE has gone missing ;)
 
J

juve021

Member
crud I was afraid of this

please elaborate on why I can't use the high side equivalent?

could I just swap the the grnd and vss/common pins on the board?
 
Last edited:
M

marks

Senior Member
What your saying may be possible i made a project board with that intention of being able to swap between a udn 2981a and a uln 2803 along as the supply is exclusively used for the darlington driver the supply would have to be reversed as you say.
heres a copy of the link so you can see how they are connected to a 20x2 hope it helps lol.
http://www.picaxeforum.co.uk/showthread.php?t=17395
 
M

marks

Senior Member
I just had a quick look at the data sheet for 030 its seems to have a common ground between the picaxe18 and the 2803 if this is the case you can not reverse the supply it will cause damage.
 
J

juve021

Member
eh forget it. I'll just use the enable pin on some LDO voltage regulators to trigger them on in a "high-side" manner.
 
M

MartinM57

Moderator
juve021 said:
Could I connect the outputs of the 2803 to a CMOS hex inverter (4049) http://focus.ti.com/lit/ds/symlink/cd4049ub.pdf , power the inverter with 12v and then connect the positive leads of the LEDs to the inverter outputs and the common ground to ground?
Click to expand...
You probably(*) could do that - good lateral thinking ;)

You would need a pullup resistor on the inverter input as the 2803 doesn't actually "output" anything - it either switches to ground (* well, not actually - maybe to within 0.5-1.2V of ground, so the inverter would have to consider that as logic low) or is open circuit.

LDO on/off is a good idea too - possibly a sledgehammer for a real product but fine for a one-off/experimental/ptototyping thing
 
M

Marcwolf

Senior Member
Have a look at a UDN2981 - It's like a ULN2803 but it is perfect for common cathode LED's such as TRI-Color units, or your buttons

http://www.datasheetcatalog.org/datasheet/allegromicrosystems/2981.pdf

It's a 8 buffer driver chip with a common ground so you can use an output from the Picaxe like such

Picaxe Out 1 -> UDN2981/1 -> LED 1 -> common ground
Picaxe Out 2 -> UDN2981/2 -> LED 2 -> common ground
Picaxe Out 3 -> UDN2981/3 -> LED 3 -> common ground

The UDN can also handle voltages higher than 5V so there is also no probs with putting 12V into it. You can also run the common ground through a MOSFET and PWM the LED's to change the brightness.

Hope this helps
Dave
 
J

juve021

Member
Marcwolf said:
Have a look at a UDN2981 - It's like a ULN2803 but it is perfect for common cathode LED's such as TRI-Color units, or your buttons

http://www.datasheetcatalog.org/datasheet/allegromicrosystems/2981.pdf

It's a 8 buffer driver chip with a common ground so you can use an output from the Picaxe like such

Picaxe Out 1 -> UDN2981/1 -> LED 1 -> common ground
Picaxe Out 2 -> UDN2981/2 -> LED 2 -> common ground
Picaxe Out 3 -> UDN2981/3 -> LED 3 -> common ground

The UDN can also handle voltages higher than 5V so there is also no probs with putting 12V into it. You can also run the common ground through a MOSFET and PWM the LED's to change the brightness.

Hope this helps
Dave
Click to expand...
OK so does the 2981 work like a pnp switch in that that if I'm trying to switch 12v with a picaxe output, I need to supply a higher voltage? Or can I indeed just use the picaxe outputs straight into the 2981 inputs and provide the 2981 with 12v to do the LED anode switching?
 
J

juve021

Member
Great! Thanks all for the great info! I will order one of these and go with it. Digikey (I'm in the USA) seems to have them (~$2 tho geesh).
 
You must log in or register to reply here.
Top