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Driving Higher Voltages from PIC 16F88

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Tom81

Member
Hi all,

I'm using a PIC to drive a 7-Segment LED circuit. The Micro can supply 5V, but each segment needs 8V/20mA to switch on and off...

Before I wire the circuit up to the micro, I've been running a few tests on a breadboard, using a 8V power supply and a 7805 5V Regulator.

What's the best way to switch a segment on/off? I tried to use a BC548 NPN Transistor, with +5V going to the base via a 1K resistor, and +8V going to the segment (via the NPN), but it doesn't seem to like that (see attached).

All the grounds are tied together, which I suspect could be the source of the problem. Any thoughts?

Please help!

Thanks,
Tom
 

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blueroomelectronics

Well-Known Member
You need both an NPN & PNP transistor per segment (should have bought CA displays)
(I couldn't find a NPN / PNP version with a quick google search) but it's similar to the image below.
 

Pommie

Well-Known Member
Most Helpful Member
You need to either,
change the NPN to a PNP and swap C and E around.
Or,
Swap the segment and the transistor, so the segment is between C and +8V.

You should also add a current limiting resistor in series with the segment.

Mike.
 

Tom81

Member
blueroomelectronics - Thanks for that. I did see a similar circuit on the net, but I was hoping to get away with less components.

Mike - I'll give it a crack when I get home. Thanks again

-Tom
 

bananasiong

New Member
You need to either,
change the NPN to a PNP and swap C and E around.
Or,
Swap the segment and the transistor, so the segment is between C and +8V.

You should also add a current limiting resistor in series with the segment.

Mike.
PNP can never be turned off with the base 5 V and the emitter 8 V.

*EDIT: For more than 5 V, use common anode with PNP coupled with NPN.
 
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Mike - K8LH

Well-Known Member
You might consider using one of the high side driver or sourcing driver ICs. Connect your 8-9 volt source to the Vs pin on the UDN2981.


Connect your 8-9 volt source to the VBB pin on an MIC5891 serial-to-parallel sourcing driver IC.

 

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Tom81

Member
Hi Mike K - circuit looks good, and straightforward too. Although I'm not sure where I can get UDN2981 at short notice. I'm sure there's an equivalent available in Oz.

As an alternative, I've got a 4511 BCD to 7-SEG Latch/decoder driver kicking around somewhere. Unbeknownst to me, turns out these can handle up to 15V. I'm thinking if I use 8~9V as Vdd to the decoder, I might be able to get away with feeding the four data pins with 0-5V straight from the Micro. Whether 5V (as a data input to the 4511) will register as a 'Logic 1' is another question... if it does, it would make life a lot easier in compenents and I'd save on port pins from the micro too. Then I can just use the NPN's as switching (as per the circuit above).
 

Tom81

Member
Thanks for everyone's input. Attached you'll find the circuit I ended up using (minus a few components). I would have liked to have used the UDN chip (I really needed to have control over each segment so I could display letters as well as numbers). I needed something sooner though, so I went with the 4511.

Some things to note about the circuit: I've omitted filtering capacitors, largely because I'm lazy. I put one either side of the Regulator, and a couple for the PIC.
For PIC timing/accuracy I used a 4MHz crystal (also not shown).
You'll need to tie some of the function pins on the 4511 to +V or ground, depending on how you use it. I can't recall which pins. Best to check the data sheet.
There are four LED-displays in total, although for simplicity, there's only one on the ciruit.

I measured my Voltage supply at 9.3V. The datasheet for the Led-display indicates 8.8V maximum supply voltage for each segment. Conveniently there's a small V-drop across the 4511 (around .8V I think), so that worked out nicely. Again, you can check the datasheet for this.

-Tom
 

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