2N7000 does this work? (in a common anode display)

I was under the impression that the Gate on a 2N7000 had to be a higher voltage than the Drain for the device to turn on.

I've attached the schematic and the full article can be found here.

http://massmind.org/techref/io/led/8x7s8pin.htm

I've used 2N7000s before but not with the drain tied to +5.

Hey Mike, it's your schematic!!! It was this schematic that inspired me to design using charlieplexing
(I don't want to pay $10 for the maxim chip even though it's a great chip)

William At MyBlueRoom said:

I was under the impression that the Gate on a 2N7000 had to be a higher voltage than the Drain for the device to turn on.

Click to expand...

No, the gate has to be at a higher voltage than the source.

The configuration uses the 2N7000s in the source follower mode. Equivalent to the emitter follower mode with BJTs.

It is a very good idea.

ljcox said:

William At MyBlueRoom said:

I was under the impression that the Gate on a 2N7000 had to be a higher voltage than the Drain for the device to turn on.

Click to expand...

No, the gate has to be at a higher voltage than the source.

The configuration uses the 2N7000s in the source follower mode. Equivalent to the emitter follower mode with BJTs.

It is a very good idea.

Click to expand...

I wouldn't have thought so?, presumably you lose considerable voltage across the FET's?, and you don't have much to lose from a 5V supply. What about using P gate ones instead?.

Nigel Goodwin said:

What about using P gate ones instead?.

Click to expand...

Can't do. With common anode 7-segment display, Charlieplexing relies on port pin goes into logic HIGH or high impedance to turn OFF a segment, and LOW to turn them ON. For those digits other than the current digit, the P type MOSFET gate would float to some voltage in between HIGH and LOW. If one output a LOW to any other port pin to turn ON a segment, the P-MOSFET whose gate on that port pin will turns ON too.

Charlieplexing is very difficult to adopt for use if LED current exceeds the driving capability of uC.

I have tried to design such an interface but failed. Perhaps others can do it better than I.

Could I put a 1N4001 diode between the +5 rail and the drain of the 2N7000s This should in theroy drop the drain vcc to 4.3V still more than enough to drive the LEDs even 2" ones.

Also I thought the voltage drop across the 2N7000 was very small, almost zero unlike a 2N3906.

William At MyBlueRoom said:

Could I put a 1N4001 diode between the +5 rail and the drain of the 2N7000s This should in theroy drop the drain vcc to 4.3V still more than enough to drive the LEDs even 2" ones.

Also I thought the voltage drop across the 2N7000 was very small, almost zero unlike a 2N3906.

Click to expand...

Nigel is talking about the gate to source voltage which is typically 2.1 V for a drain current of 1 mA. A diode in series will make it worse, not better.

It will only work for a low current display. You would need to use displays that are bright enough to read with a steady current of <3 mA since to mux 8 displays, you will need a current pulse 8 times that of the steady current. eg. if 2.5 mA is adequate for a constant current, then the pulse will have to be 20 mA.

The PIC can only sink 25 mA.

I would use NPN transistors rather than the FETs since the Vbe is about 0.7V thus providing more voltage for the LEDs and sundry voltage drops.

William At MyBlueRoom said:

I was under the impression that the Gate on a 2N7000 had to be a higher voltage than the Drain for the device to turn on.

I've attached the schematic and the full article can be found here.

http://massmind.org/techref/io/led/8x7s8pin.htm

I've used 2N7000s before but not with the drain tied to +5.

Hey Mike, it's your schematic!!! It was this schematic that inspired me to design using charlieplexing
(I don't want to pay $10 for the maxim chip even though it's a great chip)

Click to expand...

No, that schematic doesn't work... I haven't had any luck getting Mr. Newton to remove it... Here's a more up-to-date Charlieplexing Driver drawing for common anode or common cathode displays...

As mentioned, unless using low current displays from Agilent or others (2-3 ma / segment), Charlieplexed display brightness is limited by the I/O pin current spec... BTW, Charlieplexed common anode displays are brighter than common cathode displays because an I/O pin can sink more current than it can source...

Regards, Mike

I use Charlieplexing as a reduced-pin-count display solution for low current displays or for small discrete LED matrices...

For a reduced-pin-count solution using 'standard' 20-ma / segment displays I use an additional driver chip and standard multiplexing to produce a full brightness display (with PWM fade control) and a 9-pin interface... I use 16-pin Micrel MIC5821 or Allegro A6821 drivers or 18-pin Micrel MIC5841 or Allegro A6841 drivers... The PNP anode column driver transistors are 2N4403/MMBT4403 (600-ma) or PN2907A/MMBT2907A (800-ma)...

Here's one way to drive the display depicted in the drawing below;


Regards, Mike

Mike said:

William At MyBlueRoom said:

I was under the impression that the Gate on a 2N7000 had to be a higher voltage than the Drain for the device to turn on.

I've attached the schematic and the full article can be found here.

http://massmind.org/techref/io/led/8x7s8pin.htm

I've used 2N7000s before but not with the drain tied to +5.

Hey Mike, it's your schematic!!! It was this schematic that inspired me to design using charlieplexing
(I don't want to pay $10 for the maxim chip even though it's a great chip)

Click to expand...

No, that schematic doesn't work... I haven't had any luck getting Mr. Newton to remove it... Here's a more up-to-date Charlieplexing Driver drawing for common anode or common cathode displays...

As mentioned, unless using low current displays from Agilent or others (2-3 ma / segment), Charlieplexed display brightness is limited by the I/O pin current spec... BTW, Charlieplexed common anode displays are brighter than common cathode displays because an I/O pin can sink more current than it can source...

Regards, Mike

Click to expand...

Mike, I must be really dense.
How do you turn on a segment that lies on the diagonal line, e.g., segment A on the rightmost display?

Ron H said:

Mike, I must be really dense.
How do you turn on a segment that lies on the diagonal line, e.g., segment A on the rightmost display?

Click to expand...

Hi Ron,

I call that diagonal line the "float bit" or "float pin"... RB0 normally carries out segment A driver duties, that is, it's normally driven as output=1 to light segment A for a particular column (common cathode) or it's set to a hi-z input (neither sourcing or sinking current) to leave segment A unlit... When RB0 is called upon to perform its column driver function (driven low to enable a column) it can't perform its normal segment A driver duties and so we "float" the segment A driver duties onto that RA0 "float pin"...

So we use this "float" mechanism to allow each I/O pin to perform double duty as both a segment driver and as a column driver... Pretty slick, yes, no, maybe, huh?

Regards, Mike

I'm even denser than I thought I was. For some reason, I wasn't looking at the diagonal line as an actual connection. I was thinking that segment A in the rightmost column was driven by RB0. Duhhhh.... Brain fart.
Thanks, Mike.

Ron,

I apologize... That's definately a shortcoming in that drawing... I'd like to come up with a better and more intuitive drawing but "no joy" so far...

Regards, Mike

Mike said:

Ron,

I apologize... That's definately a shortcoming in that drawing... I'd like to come up with a better and more intuitive drawing but "no joy" so far...

Regards, Mike

Click to expand...

It wasn't your fault, Mike. It should have been clear to me.

Ron

Here is a display driver that uses minimal PIC i/o lines; drawn with a finger nail dipped in tar.

It is drawn for a common anode display, but a common cathode version is also possible.

It is only suitable for low current displays due to the current sinking limit of the 74LS47.

It would be possible to eliminate transistors Q1 ~ 8 if the 74HC42 was replaced by an Active High 1/10 decoder with sufficient source current.

A 3 i/o line using shift registers is possible if SRs with sufficient output sourcing/sinking are available.

Hi Len,

Your two chip solution only uses 7 pins and would be brighter than a Charlieplexed display but, as you say, the 'LS47 40-ma output current limit is probably a bit shy of the current required to drive standard current displays to full brightness in a multiplexed 12.5% duty cycle 8 digit display...

If you're going to throw chips at the problem, so to speak, in order to reduce the interface pin requirement, you might as well do it in a way to provide full brightness and more capability... I would suggest replacing the 'LS42 with an 'HC595 serial-to-parallel SR to drive the column transistors and replacing the 'LS47 with a serial '5821 or '5841 sinking driver (350-ma) for the cathode segments... The two chip 'HC595 and '5821/'5841 solution would use a 3 pin or 4 pin interface (Data, Clock, Strobe, and optional PWM), would be capable of driving 'standard current' displays to full brightness with optional PWM brightness control, and would support additional 8 digit displays (or 8x8 matrices) easily using the same 3 or 4 pin interface...

Food for thought... Kind regards, Mike

Thanks for the suggestions Mike,
I had not heard of the 5821, 5841 ICs.

They're nice chips that I normally use for relay drivers... Similar to the 8-bit ULN2803 sinking driver but with a 3-pin serial interface and built-in serial-to-parallel shift register...

Now if I could only find a serial-to-parallel 8-bit driver for high-side switching...

Take care... Have fun... Kind regards, Mike