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Digital Design Assistant


While designing low speed digital circuits, it can be a big help to have a few logic level inputs to use for testing as well as an easy way to test outputs. Of course, the outputs can be seen with a logic analyzer, but you don't have one of those lying around, then an LED is typically the way to go. With this in mind, I designed this simple digital design assistant.

Electrical Design

This is a very simple circuit. An array of eight dip switches are connected to a five volt supply with 1k pull down resistors on the opposite side of each switch. When a switch is "on" a logic level HI is seen at the terminal; otherwise a strong connection to ground is available. These switches are ideal for use as inputs to a digital circuit. To a test a circuit's outputs, two six pin headers are connected to an array of inverter powered LEDs. If the DDA input (test circuit output) is HI, then the inverter output is LO creating a current sink for the green LED. If the DDA input is LO, then the inverter output is HI creating a current source for the red LED. With this arrangement, current is only flowing through one of the LEDs at a time. Also, with no input applied to the gate of the inverter, there is no way for the LEDs to turn on in the idle state due to the high impedance input of the inverter, even if the logic gate behaves erratically or has a default output state of HI or LO. I chose 430Ω resistors to limit the LED current primarily because I had a lot of them in stock.

Of some note is the power supply design. Although the circuit is powered by a common 7805 regulator to drop the input voltage down to five, a third input pin is tied directly to the VCC supply rail so that the entire DDA circuit can easily be powered from the circuit it is testing. This was added after I built the circuit and realized it would be a lot more useful if it didn't need it's own battery for power.

Physical Design

As always, I tried to create a circuit as visually appealing as it is effective. All of the parts were soldered to a small prototype board with the I/O headers and dip switches together, and the LEDs grouped in two straight rows. Although I normally use IC sockets to prevent overheating chips while I am soldering, I took a risk and did my best to not destroy the two 7404 chips used to turn the LEDs on and off. It payed off as I was able to save two sockets without cooking these chips. With what little time this circuit is powered, I doubt that they will need to be replaced within the usable time of this circuit anyway unless I try to input some stupidly high voltage. And if that is happening, I've probably lost all need for such a circuit anyway.

The original prototype circuit.

Prototype circuit with the modified power supply input.