Logistics of wiring the drains

Thread Starter

pdavis68

Joined Nov 27, 2013
37
To begin, yeah, I probably should have figured this out before I soldered it all together. I had some ideas but I never was real happy with them and so I haven't done anything about it.

So I've got this strip board I'm using as an address bus and another that I plan on using as a data bus. The idea is the address lines come in on the SIP pins (16 for the address bus. 6502 CPU). On the south side of the bar graph LEDs are a couple of terminating resistors for the LEDs.

My plan was to use 2n7000s to drive the LEDS (because I've got a bag of 100 of them lying around). So I'd do the gate on the bus side, the source on the LED side and then I need to connect all the drains to power.

So that's my question. From a logistics point of view, how best can I do that without short circuiting anything. So I've got a break on the under-side that I planned to span with the gate and source legs of the 2n7000s. I thought about running a stiff, bare wire with the voltage above the PCB spanning the top that I'd tack each drain to in a point-to-point style. Having the voltage bare and exposed just seems BAD(c). I've done point to point in vacuum tube amps, but seems wrong with a digital build.

Anyone have some better ideas for how to do this?
 

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sghioto

Joined Dec 31, 2017
1,469
Can you post a schematic? From what I've read I think you should connect the source to ground and the LED to the drain, other side of LEDs to power.
SG
 

AnalogKid

Joined Aug 1, 2013
8,606
Might not work. You are using the FETs as source followers so they do not invert the logic polarity (high = LED on). But starting from GND you have the LED forward voltage (Vf), then the FET threshold voltage (Vth). The LED current limiter can be connected from the drain to Vcc, but some TTL backplanes have a standard high signal voltage of around 3 V (example: 2.93 V for VME). The bus address signal might not go high enough to turn on the FET. A bipolar transistor (2N3904, 2N4401) will work, but draw more current from the bus.

An alternative to to change the FETs to act as saturated switches:

Gate - bus signal
Source - GND
Drain - LED and current limiting resistor

ak
 

Thread Starter

pdavis68

Joined Nov 27, 2013
37
Might not work. You are using the FETs as source followers so they do not invert the logic polarity (high = LED on). But starting from GND you have the LED forward voltage (Vf), then the FET threshold voltage (Vth). The LED current limiter can be connected from the drain to Vcc, but some TTL backplanes have a standard high signal voltage of around 3 V (example: 2.93 V for VME). The bus address signal might not go high enough to turn on the FET. A bipolar transistor (2N3904, 2N4401) will work, but draw more current from the bus.

An alternative to to change the FETs to act as saturated switches:

Gate - bus signal
Source - GND
Drain - LED and current limiting resistor

ak
I didn't consider that about the LED voltage. Shoot. Maybe best to just make a new board and not do it as a source follower.
So if I did it that way, as a common source amp, I'd still be left with the same logistical issue, except it would be how to tie all the sources to ground.
 

Thread Starter

pdavis68

Joined Nov 27, 2013
37
Might not work. You are using the FETs as source followers so they do not invert the logic polarity (high = LED on). But starting from GND you have the LED forward voltage (Vf), then the FET threshold voltage (Vth). The LED current limiter can be connected from the drain to Vcc, but some TTL backplanes have a standard high signal voltage of around 3 V (example: 2.93 V for VME). The bus address signal might not go high enough to turn on the FET. A bipolar transistor (2N3904, 2N4401) will work, but draw more current from the bus.

An alternative to to change the FETs to act as saturated switches:

Gate - bus signal
Source - GND
Drain - LED and current limiting resistor

ak
Thanks for the help. In re-thinking this, I believe what I can do is simply cut the board between the LEDs and the SIP connectors. I can add a row of SIP connectors to the LED side and then on a separate board get some 74LS573s set up in between to drive the LEDs.
 

MrChips

Joined Oct 2, 2009
21,880
2N7000 is fine.
However, configure it with source pin to ground and LED on the drain side, just like a BJT with emitter to ground and LED on the collector side.
Choose the value of the current limiting resistor R2 for suitable brightness.

1600557947783.png
 
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