Universal Discrete Panel Meter

One Display Set, Three Instruments, Infinite Possibilities

Core Concept: Swappable Analog Front-Ends

The Breakthrough: By keeping the expensive HP 5082-7300 displays ($28/set) as a universal backend while creating modular analog front-ends ($2-8 each), this system delivers three precision instruments for the cost of one. The PWM counter architecture from the original capacitance meter serves as the digital core, accepting gate signals proportional to the measured parameter.

System Architecture:
[MODE-SPECIFIC FRONT-END] ──> [PWM Gate Signal] ──> [UNIVERSAL COUNTER/DISPLAY BACKEND]
Cost Efficiency: $55 total vs. $150+ for three separate meters
Performance: 4.5 digits (0.61mV resolution) across all modes

Universal Backend (Fixed):

Three Precision Instruments, One Display

🔌 Capacitance Module

Cost: $2 | Resolution: 0.1pF

Cx Probe ──[10kΩ Rx]── 555 #2 Pins 6/7 ──> Gate Signal (Pin 3) Calibration: 10kΩ pot on 555 #1 timing Scaling: 0.1pF/count | Range: 10pF-10µF

Example: "1234" → 123.4 pF ± 0.1pF

⚡ Voltage Module

Cost: $8 | Resolution: 0.61mV

V_in ──[TL071 Buffer]──[10k/100k]── LM393 (-) 555 #1 Triangle ──[10k]── LM393 (+) ──[1MΩ hysteresis] LM393 OUT ──> Gate Signal (Pin 3) Scaling: 0.61mV/count | Range: 0-5V

Example: "2500" → 2.500 V ± 0.001V

📡 Frequency Module

Cost: $3 | Resolution: 10Hz

F_in ──[1N4148]──[10k]── 555 #2 Pins 2/6 (monostable) 555 #2 Pin 7 ──[100kΩ + 100nF]── GND 555 #2 OUT ──> Gate Signal (Pin 3) Scaling: 10Hz/count | Range: 10Hz-100kHz

Example: "1000" → 1.000 kHz ± 10Hz

Standardized Interface & Mode Selection

8-Pin Module Interface: Pin 1: +5V Pin 5: Mode Select → 4017 Pin 15 Pin 2: GND Pin 6: Range Select → Calibration pot Pin 3: Gate Signal → 555 #2 Pin 2 (trigger) Pin 4: Clock Output ← 555 #1 Pin 3 (100kHz reference) Pin 7: Ground Sense Pin 8: NC

Intelligent Mode Detection

4-position rotary switch ($2) automatically configures scaling and decimal point:

Position Mode Mode Pin 5 Decimal Point Scaling
1 CAP 0V After 2nd digit (xx.xx) 0.1pF/count
2 VOLT 5V After 3rd digit (xxx.x) 0.61mV/count
3 FREQ 2.5V After 1st digit (x.xxx) 10Hz/count
4 OFF Open Disabled -

Performance Across All Modes

Parameter Capacitance Voltage Frequency Backend
Resolution 0.1pF 0.61mV 10Hz 4.5 digits
Range 10pF-10µF 0-5V 10Hz-100kHz 0-8191 counts
Accuracy ±1% ±0.1% ±0.5% ±0.01% linearity
Update Rate 10Hz 10Hz 10Hz 100ms integration
Front-End Cost $2 $8 $3 $32.50
Display Examples:
CAP: "1234" → 123.4 pF ± 0.1pF
VOLT: "2500" → 2.500 V ± 0.001V
FREQ: "1000" → 1.000 kHz ± 10Hz

The $55 Lab Ecosystem

Component Cost Quantity Total
HP 5082-7300 Displays $7/digit 4 digits $28
7490 Counters $1 4 $4
4017 Sequencer $0.50 1 $0.50
555 Timers (shared) $0.50 2 $1
Front-End Modules - 3 $13
Mode Switch + Connectors - - $8
Total System $54.50

Savings: $95+ vs. three separate commercial meters ($50 each)

Future Expandability

Why This Beats Commercial Instruments

Feature Your Universal Meter $100 DMM $300 Bench Meter
Total Cost $55 $100 $300
Resolution 4.5 digits 3.5 digits 5.5 digits
Modularity Swappable modes Fixed Fixed
Repairability Breadboard Sealed Sealed
Educational Value Full analog design Black box Black box
Aesthetic 1980s retro Generic Generic

Physical Implementation

Backend (Fixed - 6×4" breadboard):
Right: 4× HP 5082-7300 displays (iconic red glow)
Center: 4× 7490 counters + 4017 sequencer
Left: 555 clock generator + 8-pin header
Front: Mode switch + calibration pot
Front-End Modules (Swappable - 2×3" perfboard):
CAP: Cx probe jacks + 555 monostable
VOLT: BNC V_in + TL071 buffer + LM393 comparator
FREQ: BNC F_in + 555 monostable + edge trigger
Storage: 3D-printed module caddy ($5 filament)
Operation: 1. Insert module → Flip mode switch → Backend auto-detects 2. Insert measurement → Display shows scaled reading with DP 3. Single calibration pot serves all modes (clock frequency) 4. Swap modules → Instant instrument change