Flowmeter circuit board

In the electronics and soldering workshop I built and tested a flowmeter circuit that turns a paddlewheel’s rotation into a DC voltage proportional to its speed.

Year: Year 1
Module: Engineering Practice XE421
Period: Nov 2024
Team: Individual

Assembled green circuit board labelled Flow meter V4, with two chips in sockets, a trimmer potentiometer, a red LED, resistors and capacitors, and a 9 V battery clip — My flowmeter board: an infrared sensor circuit with an LM741 comparator and an LM2907 frequency-to-voltage converter.

Components hand-soldered on the board

ICs: infrared sensor, comparator, frequency-to-voltage converter

9 V

Supply, checked at each IC before the sensor went on

How it works

An infrared reflective sensor (OPB706B) faces a rotating paddlewheel with black and white segments, and produces a pulse each time a segment passes. An LM741 op-amp, set up as a comparator, squares these into clean pulses, and an LM2907 frequency-to-voltage converter turns the pulse rate into a DC voltage proportional to the wheel’s speed, and so to the flow.

What I did

The workshop was on 26 November 2024.

Identified the parts. I read the resistor colour bands and capacitor codes, and found each capacitor’s polarity from its lead length: 19 parts in all, including the three ICs, a 10 kΩ trimmer and an LED.
Soldered the board. I soldered the components, with DIL sockets for the two chips.
Checked it before powering the chips. I measured the resistance across the supply to check for short circuits, then checked the LED and the 9 V supply at both IC sockets.
Fitted and tested the sensor. I soldered in the opto-sensor and checked its output waveform on the oscilloscope while a motor turned the paddlewheel.

The full procedure also checks the current draw (about 0.04 A at 9 V; 0.02 A points to an LED or sensor fault) and sets the trimmer so that the comparator output swings from nearly 0 V to nearly 9 V.

Test rig: the board mounted above a motor-driven wheel on a wooden base, with oscilloscope probes attached, in front of a bench power supply and an oscilloscope — Testing my board: a bench supply, the motor-driven paddlewheel and an oscilloscope to check the sensor’s square-wave output.

A soldering iron working on the track side of the flowmeter circuit board — Soldering the track side of my board.

Looking back

The build went step by step, with the board checked at each stage before the next went on. But my write-up leaned on the workshop’s support material and recorded almost none of my own results, and it had technical slips.

No readings. I didn’t record my measured resistance, current, pin voltages or output voltage, or capture the oscilloscope trace, so the report can’t show that my board worked.
A wrong number. I wrote that the supply resistance should be 5–7 Ω, but the procedure says 5–6 kΩ.
Wrong safety advice. I blamed the hazard on lead fumes and suggested holding your breath while soldering. The real hazard is rosin flux fume, a leading cause of occupational asthma, and the control is fume extraction, keeping your head out of the plume.
A confused voltage. I said the voltage across the LED is about 9 V. That’s the supply; a red LED drops about 2 V.

What I’d do differently

Record every reading against the expected value: resistance, current, pin voltages and output.
Capture the oscilloscope trace and plot output voltage against wheel speed, to show the converter is linear.
Write the safety section from HSE guidance rather than the support pack.