Designing a Multimeter

Designing a Multimeter

1. Functions

   • Frequency Counter
     • Use of Schmitt trigger inverter (datasheet below) allows for measurement of triangular and sinusoidal waves
     • Could display periodic time alongside frequency
     • Max frequency limited by the hardware , ~30 MHz is typical max for digital electronics - warning must be coded in for when frequency exceeds this
     • Code should recognise when to display Hz, kHz, MHz
     • Multiple possibilities for measuring periodic time
       • Option 2: Measure time spent at logic high and add to time spent at logic low
       • Option 1: Start timer the instant logic high is reached, stop when reached again
       • Average of 6 or 7 frequency values calculated for accuracy, regardless of method
   • Voltmeter
     • Voltage limit of board is 3.3 V - use potential divider circuit to scale down
       • Ratio of R1 to R2 is known - determines scale factor
       • May be possible to share this circuit with ohmmeter to cut down on space/cost?
     • Voltage measured in parallel - use probes
     • Must measure voltage up to ±15 V
   • Ohmmeter
     • Known resistor can be varied depending on order of magnitude of unknown resistor - implements autoranging feature
     • Resistance measured through a material
     • Use potential divider circuit
       • Resistor ratio unknown, voltage in/out ratio known
     • Has to measure from 10 Ω to 10 MΩ
   • Capacitance Meter
     • R2 = 220 Ohms - to limit current into Nucleo
     • Must measure capacitance from nanofarad to microfarad range
       • Code should account for order of magnitude when displaying capacitance
       • Could average out multiple readings for greater accuracy provided charge times aren't too long
     • Functions based on RC time constant - 63.2% charged at 1 τ and C = R/τ
       • Resistance of circuit is known, time constant found by using timer() function
     • Vc monitored consistently - could display charge status on LCD for user?
     • R1 is multiple resistors in parallel - autoranging depending on capacitance
   • Display
     • LCD used as more information can be displayed in smaller space
     • LCD allows for more legible display of units
     • Backlight requires its own 220 ohm resistor
     • Requires ~8 digital pins among others - may necessitate multiplexing
     • Codes in development can be trialled on serial monitor until LCD has been obtained and soldered

2. Other aspects to consider

   • Safety
     • Reject high voltage inputs, display a warning which states max input
     • Keep positive and negative potential leads as far away from each other as design allows
     • Insert fuse/circuit breaker to protect circuit from excess current and overheating
     • Insulate entire multimeter from its surroundings
     • Protect against voltage spikes (transients) - double insulated leads
   • Precision
     • Scaling of voltage for circuits which increase voltage swing or shift the high/low logic level
       • Margins of error must be scaled with voltage
     • Known resistances - factor in resistances of wires, ADC converter and any other components used
     • Factor in losses in voltage across components, energy dissipated through resistors and through heat loss
       • Double insulated wires might make the heat losses negligible
     • Calibrate instrument using known multimeter
   • Team Organisation
     • Also divide based on preference where possible
     • Divide tasks based on expertise
     • Break tasks down into key ideas
   • Business Case
     • Demand for this instrument from technicians in electrical/electronic industries
       • Used to repair household/industrial appliances
       • Industrial/commercial troubleshooting, installation, maintenance etc
       • Troubleshoots a wide variety of measurement parameters
     • Building your own is a cheap alternative to buying a mid-upper range multimeter
       • Could sell as kit and market to students
     • Essential tool in technology-reliant society, but faces crowded market
       • Lacks features available in other multimeters...
         • Bluetooth/smartphone connectivity
         • Ammeter
         • Wireless screens
         • Compact/industrial size
       • Added functionality/larger ranges drive up cost - less marketable
   • Lifecycle
     • Must comply with Restriction of Hazardous Substances - 10 restricted substances cannot be used
     • Could offer lifetime guarantee on product
       • Sell in case that makes it shockproof, waterproof, dustproof etc
       • Lifetime guarantee raises manufacturing and therefore retail cost
       • Adding fuses and other safety measures to protect against misuse raises price but prolongs life of the product
     • 9 million tonnes of electrical and electronic waste in 2005 according to WEEE - needs to be reduced
       • Design a product with warranty
       • WEEE offers free collection at collection centres
       • Must handle disposal of product responsibly when it cannot be used anymore
   • Cost
     • Have multifunctional circuits where possible - fewer components = cheaper
     • Cost of soldering connections should be considered
     • Limited memory in microcontroller - concise code so that extra memory is not required
     • Tradeoffs occur between cost and...
       • Resistor tolerance
         • Conversely, a more precise multimeter has a higher retail value to offset manufacturing cost
       • Range that can be measured
       • Size
       • Safety (once regulations are met)

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