Logic Tutor

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Logic Tutor with patch leads

The Logic Tutor is an educational toy that lets you quickly experiment with simple digital logic circuits. It features:

  • nine three-input NAND gates
  • six inverters
  • four D-type flip-flops
  • four switches
  • a push-button
  • four LEDs
  • a clock generator

All inputs and outputs are brought out to pins and can be connected at will using patch leads.

The design is inspired by a device that came (I think) as part of the practical materials accompanying the Open University T100 technology foundation course in the 1970s. Information about it on the Internet is sadly sparse, though there is evidence in the form of library catalogue entries for a ‘Logic tutor handbook’ by John Jackson Sparkes. Any further information would be welcomed at the e-mail address on the home page. I would be particularly interested to see a photograph of the original unit.

The circuit

Circuit diagram

Download a PDF version of the circuit diagram here.

The circuit uses 74HC logic for low power consumption and can be powered from two AAA cells providing 3 V. The design includes rudimentary protection against the short-circuiting of outputs in the form of a series resistor in the supply to the logic: this is possible because the normal operating current of the circuit is orders of magnitude less than the short-circuit current needed to damage a device. The LEDs, which can draw considerably more current, are powered directly.

All gate inputs have pull-up resistors so that, for example, a three-input NAND gate can be used as a two-input NAND gate by simply leaving one input unconnected. Likewise, the inputs to the flip-flops are pulled to their inactive levels. The clock generator output has a series resistor to reduce ringing, and the switches and push-button are debounced.

None of the component values is particularly critical, with the exception of C17 and R54, which determine the clock generator frequency. With the values shown the frequency is around 0.5 Hz.

PCB design

PCB layout

Gerber files are available here. They are made available under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License (CC BY-NC-SA 3.0).

The board is designed to fit a CamdenBoss BIM2003/13 enclosure (available here, for example), replacing its lid, component-side down. You will need to add a battery and a power switch and replace the lid fixing screws with non-countersunk types. The SPDT switches are this type and the (changeover) pushbutton is this type.

Most of the components are surface-mounted on the top of the board. The pins are standard 2.54 mm/0.1 inch headers mounted through-hole on the bottom of the board. Using standard header pins allows low-cost female-to-female ‘Dupont’ cables to be pressed into service as patch leads. The 3 mm LEDs are fitted through the board and soldered to pads. Flying leads connect the pins of the switches and the push-button to further pads. The silk screen on the bottom side of the PCB provides the front panel legend.

The logic ICs are all in SOIC-14 packages and most of the passives are in 0603 packages. It is possible to hand-solder the board, but there are areas where the components are densely packed and reflow soldering is overall a better option if available.

The PCB is double-sided, but for neatness the only tracks on the bottom layer (i.e., the front panel) are those that link groups of pins joined to the same node. These tracks do form part of the circuit—a fully single-sided design may well be impossible—so the holes for the pins must be plated through, or else the pins can be soldered on both sides.

I have a couple of spare unpopulated printed circuit boards like the one shown in the pictures available in exchange for a donation to a charity as indicated in the bar to the right. Please get in touch if you are interested via the e-mail address on the home page.

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This page most recently updated Fri 4 Feb 16:49:54 GMT 2022
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