Hello-
I have a gate circuit built using a SN74AC32 and an SN74AC14. It has 2 inputs that switch 4 resistive optocoupler LED's on and off to control audio signals. I also want to use it with an external switch that connects via a .25" TRS jack. The external unit has 2 more LED's with 1k current limiting resistors built into it. I tested these LED's and they each drop about 2 volts, and they need at least 3 mA of current to be bright enough for my liking. Also, they depend on their voltage being delivered through either the tip or ring of the TRS jack. They will be in series with the pull-up resistors for the gate inputs. The 5 volts I was feeding them wasn't carrying enough current to light them up properly. Additionally, the voltage drop when they were (barely) conducting was such that my gate inputs didn't interpret the high signal correctly.
Would it be easier or simpler to just build another higher voltage supply rail to run these LED's, or is there something better? These LED's only need 6 mA each, and the entire rest of the circuit will draw about 50 mA at most. This is my first attempt at a logic circuit, and my first post here.
Any insight is appreciated!
Colin D.

 

Post your circuit, if it's just the current draw that's an issue then use a better psu.

 

LEDs with 1kΩ current limiting resistors should draw about 3mA with 5V supply. 74AC series should be able to drive LEDs.

The general recommendation is to drive the LEDs using gates such as 74AC04, 74AC14, to buffer (i.e. isolate) the signal from the original logic source.

Do not use the output of the buffer (i.e. where the LED is driven) as a logic source for another logic input. The reason for this is that the LED load will degrade the logic level output voltages into unacceptable ranges.

 

Welcome to AAC!

Thank you for trying to organize your thoughts in paragraphs. A blank line between paragraphs would help as your post looked "dense", i.e. single paragraph, on a tablet.

Schematics are always helpful when discussing circuits; as would a description of your power source since you're having a problem with current.

Would it be easier or simpler to just build another higher voltage supply rail to run these LED's

The gate would still have to sink the current, so there's little point in doing that.

These LED's only need 6 mA each, and the entire rest of the circuit will draw about 50 mA at most.

74AC is specified to sink/source 24mA. Sinking or sourcing 6mA should still allow correct logic level recognition.

 

Here's my schematic:
https://drive.google.com/file/d/0B3XMEDN5DU-JSE5XLTI1ajVVUVU/view?usp=drivesdk
It doesn't show the pull up resistors on pins 1 and 2 of J3.

And here's the external unit:
https://drive.google.com/file/d/0B3XMEDN5DU-JUmd2a0RtX0lnS0E/view?usp=drivesdk

If you look at the external switch drawing, I have conductor 2 grounded at the moment. Conductors 1 and 3 each have a pull up resistor to a vcc node, and are thus providing my gate input signals. There's no problem with the low level gate input signals, but the LED's in the external switch have their 2v drop and 1k current limiter, plus another 1k pull up resistor in series with the LED.

The external switch connects via a shorting .25" stereo jack that overrides a pair of SPST switches. If the external switch isn't plugged into the jack, it's LED's are no longer part of the circuit and everything functions as expected.

This is all part of a guitar amp. The external switch wss designed for use with mechanical relays, and it works well with them, but I really want to get it working with something that draws less current and takes up less space. Relays for audio switching have their own quirks as well.

Thanks!