Build Your Own Simple Transistor Based TTL 5V Logic Probe

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1024MAK
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Build Your Own Simple Transistor Based TTL 5V Logic Probe

Post by 1024MAK »

Build Your Own Simple Transistor Based TTL 5V Logic Probe

Parts required
Resistors (all ¼W / 250mW, 5% or better)
R1 - 3.9kΩ
R2 - 10kΩ
R3 - 3.3kΩ
R4 - 1kΩ
R5 - 10kΩ
R6 - 470Ω
R7 - 470Ω

Semiconductors
LED, red
LED, green
Q1, Q2, Q3 - 2N3904 or any general purpose NPN signal transistor. Other transistors that should be suitable include 2N2222, BC107, BC108, BC109, BC547, BC548, BC549, plus many others. Note that other types of transistors may have different lead outs so always check.

Schematic circuit diagram
Build Your Own Simple Transistor Based TTL 5V Logic Probe
Build Your Own Simple Transistor Based TTL 5V Logic Probe

Photos of one constructed on breadboard
Breadboard #1
Breadboard #1
Breadboard #2
Breadboard #2
Breadboard #3
Breadboard #3
Breadboard #4
Breadboard #4

Usage
Preferably use a commercially made probe.

The 0V/GND of the logic probe must be connected to the 0V/GND of the circuit under test. On a ZX80, ZX81, TS1000, ZX Spectrum 16K/48K/+/128K/+2 I suggest attaching to the heatsink for the 5V voltage regulator (7805 or equivalent) or to the RF/VHF/UHF Phono/RCA outer or the modulator case. Alternatively use the negative lead of a capacitor that connects to the 0V/GND rail.

With no signal, or when not connected to a circuit, the green LED will light. So it’s important to note that if there is a poor connection between the probe and the leg/pin under test, the green LED will light regardless of the signal on the leg/pin. So make sure you get a good connection. But don’t let the probe slip and short out adjacent legs/pins.

Before using each day, test that the red LED lights and the green LED extinguishes when the probe is touched on a +5V supply point.

The Logic Probe requires a regulated +5V DC supply. You can either use a separate mains power supply that has isolated outputs (make sure that you use a type that DOES NOT have either the positive or the negative outputs connected to mains earth). It can also be powered from the circuit under test, or from four 1.2V NiMH cells (battery).

Limitations
The performance of this simple logic probe partly depends on the transistors used.
It works reasonably well with cheap 2N3904 transistors purchased from China via eBay.
It’s frequently response is not good enough to test the CPU clock pin.
It does impose a load on the circuit being tested, so should not be used on high impedance circuits, like the /RESET circuit or the ULA oscillator pin.

Credits
Original circuit design from the book Instrumentation and Test Gear Circuits Manual by R. M. Marston

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
1024MAK
Posts: 5103
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Build Your Own Simple Transistor Based TTL 5V Logic Probe

Post by 1024MAK »

Actual Testing on a ZX81

Z80 CPU
Pin - Function
1 - A11 - both red and green lit
2 - A12 - both red and green lit
3 - A13 - both red and green lit
4 - A14 - both red and green lit
5 - A15 - both red and green lit
6 - clock - with the simple logic probe, only red will light
7 - D4 - both red and green lit
8 - D3 - both red and green lit
9 - D5 - both red and green lit
10 - D6 - both red and green lit
11 +5V - red lit
12 - D2 - both red and green lit
13 - D7 - both red and green lit
14 - D0 - both red and green lit
15 - D1 - both red and green lit
16 - /INT - both red and green lit
17 - /NMI - no need to test
18 - /HALT - red lit
19 - /MREQ - both red and green lit
20 - /IORQ - red lit
21 - /RD - both red and green lit
22 - /WR - red lit (green may sometimes light dimly)
23 - /BUSAK - red lit
24 - /WAIT - both red and green lit
25 - /BUSRQ - red lit
26 - /RESET - don’t test with the simple logic probe
27 - /M1 - both red and green lit
28 - /RFSH - red lit (green may sometimes light dimly)
29 - GND - green
30 - A0 - both red and green lit
31 - A1 - both red and green lit
32 - A2 - both red and green lit
33 - A3 - both red and green lit
34 - A4 - both red and green lit
35 - A5 - both red and green lit
36 - A6 - both red and green lit
37 - A7 - both red and green lit
38 - A8 - both red and green lit
39 - A9 - both red and green lit
40 - A10 - both red and green lit

Some of the pins have a duty cycle where they are logic high for longer than the logic low time. So the relative brightness of the red and green LEDs will vary. Typically the green LED will appear dimmer compared to normal.

If your results are not the same as above, please reset the computer and retest the pin or pins that are different again.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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