LED Driver and Output Board for SEQv4 DOUT

MIDIbox SEQ
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I know this topic has been raised before, and I’ve read through some of those threads, but I haven’t found a definite answer and couldn’t find Altitude’s files on the wiki to use as reference.  I saw that Hex Inverter’s MIDI2CV was mentioned as an example of a usable LED driver.  The specs of the MIDI2CV quote a gate output of 0-5V.   Some research proved that this is a common method of LED driving, and of course that there is plenty of debate about which method is best. 

I have designed an 8-output board based on that, but I had D0-D7 reversed on my first batch.  Before I order new boards, and as I’m not an EE, I figure it’s best to confess my ignorance and get some input on the circuit from some other MBHP builders here before I have these fabricated.  Below is a schematic of the circuit, one instance per output.  The 220Rs are already installed on my DOUT board, so I’ve included them in the sketch.    The 33k is based on the original circuit, but I changed his 1K at the collector to 150R (based on a basic LED-resistor calculation) and plan to use some of the 220s I have left over instead…but this doesn’t account for other variables (the 33k, the 3904) because I honestly don’t understand the equations.  

Here’s the datasheet for the 3904 http://www.mouser.com/ds/2/308/2N3904-1118515.pdf

I tried this circuit in a simulator with various resistor values.  I was attempting to tune it for Vbe saturation per someone’s advice, but I don’t know what I’m doing.  If I change R2 to 18K, I get the following results, but I’m not sure that matches my targets from the datasheet (again, I don’t pretend to have a grasp of this yet).

 

What are most people doing for their gate/trig/clock outs?  Sockets wired directly to J3-6 on DOUT with no LED or output protection?  Am I totally off track?

Thanks in advance for any advice!  

 

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1 Like
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Can you post the actual circuit diagram you’re going off? 

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57 minutes ago, latigid on said:

Can you post the actual circuit diagram you’re going off? 

 

 

 

 

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NPN transistors don’t source current very well. I’d suggest moving the diode in series with the resistor on the collector and tying the emitter to 0V. This way the current can be controlled with R1, otherwise it’s not doing much. The capacitor you’ve shown in the Falstad sim won’t do anything. R3 isn’t necessary if you have the 220R in place. D1 will block reverse voltages but will also drop the gate voltage. Voltage < 0V will pass through. As long as the current is limited the 595 output will probably be okay with another output connected, even excluding D1. Diode clamps are better done in parallel with the node to protect, diodes connected e.g. to the power rails.

I’d suggest testing the circuit on a breadboard or perfboard/stripboard before ordering the PCBs.

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Thanks for your reply.  The sim works fine with the configuration you suggest, but I haven’t tried much on the breadboard, haven’t had time and my SEQv4 and DOUT are not ready to use for testing.  Part of the reason I had the LED on the emitter is because I’m trying to salvage some of the version 1 boards which already have sockets and LEDs installed, so adding a daughter board with the drivers would save me from desoldering or scrapping those parts. I had figured out a way to send signals back and forth to the daughter using existing headers, but I can’t go to and from the LEDs, only one or the other.  So R1 doesn’t limit current in the sim above?  There is certainly a change in LED brightness when changing R values.

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On 10/30/2017 at 7:09 PM, latigid on said:

 

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https://imgur.com/a/dwaIr

made this quick GIF to show the breakout box and LED drivers in action.