Cheap oscilloscope and sensor probes for physics experiment in school

Wichit Sirichote,

Build a cheap digital storage oscilloscope and sensor probes for many physics experiments in school.

A prototype of cheap oscilloscope and sensors probes have been developed. The oscilloscope circuit is based on the Jyetech DSO 062, DIY oscilloscope. The hardware was modified with bigger display, adding the sensors power terminal, modified negative power supply and modified USB interface chip. The homemade sensor probes are 1) voltage probe, 2) light probe, 3) IR receiver probe and 4) sound probe. The oscilloscope and sensor probes were used in the hands-on workshop for physics teachers and school students. The article describes hardware for scope and sensor probes. The example for many physics experiment are also presented.
Figure 1: Prototype cheap oscilloscope and sensor probe.

Hardware The main MCU is AVR mega64, 64-pin TQFP. The ADC chip is Texas Instrument TLC5510 8-bit Flash converter. The display is 128x64 pixels graphic LCD.  
Figure 2: Schematic for AVR MCU, ADC, LCD display, and keypad. 

The input signal is selected by SW3 slide switch. SW4 selects the voltage level x0.1V or x1V. SW4 selects range for x1V, x2V and x5V. U2, NE5532 is for signal level amplification providing proper signal level for ADC input. The counter input signal is passed to U3C. U5 provides negative power supply for the analog circuit.
Figure 3: Schematic for analog frontend and power supply. 

The USB interface chip is FT232. The VCCIO is set to +5V the same as AVR supply.
Figure 4: USB interface.



IC1 ATmega64 AVR Flash microcontroller 64-pin TQFP
U1 TLC5510 Texas Instrument 8-bit Flash ADC
U2 NE5532 high speed operational amplifier
U3 TL084A FET input quad operational amplifier
U4 LM117A/TO +5V low dropout voltage regulator
U5 ICL7660 voltage converter
U6 FT232RL USB-UART converter

D1 LM336-2.5V/SO
D2 1N5225A
D3 1N4007
D4 1N5349A

Resistors (all resistors are 1/4W +/-5%)

R1 120
R2,R5,R8 1k
R3,R4 470
R28,R6 100k
R7 2k
R9,R12,R21,R25,R26,R27, 10k
R10 4.7k
R11 10
R13 909k1%
R14 100k1%
R15,R19,R20,R22 470 1%
R16 240 1%
R17 150 1%
R18 499 1%
R23 100 1%
R24 249 1%
R30 3.3k
R32 300

C1,C3,C4,C5,C6,C8,C12, 100nF SMD 0805 chip cap.
C14,C18,C19,C23,C24, 100nF SMD 0805 chip cap.
C2 300pF, ceramic disc
C7,C15,C16,C21,C22 10uF, electrolytic capacitor
C9 680pF, ceramic disc
C11,C10 22pF, SMD 0805 chip cap
C13 100nF 100V, ceramic disc
C17 100uF, electrolytic cap
C20 22uF, electrolytic cap
C25 10nF, SMD 0805 chip cap.

Additional parts
S1,S2,S3,S4,S5,S6,S7,S8,S9 12mm tact switch
Y1 20MHz
128x64 Graphic LCD

Software The scope's firmware can be downloaded from Jyetech website directly. The firmware is loaded into the MCU by using USBASP V2.0 and the PC software for hex code loading is eXtreamBurner.

Making sensor probes

All of the sensor probes are homemade. The materials are common parts and cheap.

1) Voltage probe
The probe can be built with 500mm stereo cable, RCA jack and two alligator clips.

2) Light probe The light detector is made with phototransistor Q1. R1 is load current. The voltage developed across R1 is increased with light intensity.
Figure 5: light probe made with phototransistor.

The light detector is built into the PVC 3/8" pipe. Details of dimension and materials required for probe housing are describes in the wokrshop handout in below link. The back color RCA jack is for signal to CH1 scope and red color for +5V supply.
Figure 6: Light probe.

3) IR remote control receiver probe The detector is made with IR receiver module, HM338.
Figure 7: IR remote control receiver circuit.

The face of detector is fixed using glue-gun.
Figure 8: IR detector is fixed and built into the PVC pipe.

3) Sound probe The sound pressure detector is made with electret microphone. The preamplifier is LM358 single supply inverting amplifier.
Figure 9: Sound probe circuit.

Figure 10: Sound probe.

More sensor porbes are available here, probes.htm

Examples of using sensor probes

Detecting AC voltage with voltage probe For safety, teacher may use the 220V:12V stepdown isolation transformer. The voltage divider, R1 10k provides amplitude adjustment.
Figure 11: Isolation transformer with 10k voltage divider.

Figure 12: Detecting AC voltage.

Converting AC to DC voltage using diode The circuit is halfwave rectifier. D1 is 1N4007 rectifying diode. R2 is load.
Figure 13: Halfwave rectifier circuit.

The detected waveform is only positive cycle.

Figure 14: Detecting voltage waveform of the halfwave rectifier circuit.

 To make more stable DC voltage source, we add the capacitor, C2 to be a charge reservoir.
Figure 15: Add filter capacitor.

The voltage across capacitor is now more smooth but still has small ripple voltage. Student may observe the waveform when adding more capacitors.

Figure 16: Ripple voltage.

Charging capacitor The circuit is made with R1, 500Ohms and C1 unknown capacitance.

Figure 17: Charging capacitor circuit.

First discharge the capacitor, then press S1 to charge the capacitor with +5V scope supply. The result waveform is shown in Figure 18.

Figure 18: Charging capacitor waveform, the RC time constant is 0.24s. From known resistance, student can find the capacitance easily.

Detecting light using light probe Teachers may provide many light sources, and use light probe to detect the light pulse.
Figure 19: Detecting light pulse from light source .

If the source is Neon lamp, we still detect the light pulse? Observe the waveshape.
Figure 20: See the wave-shape of Neon lamp.

Detecting remote control signal Many remote control transmitters can be used. The example was remote control for Sony TV. The detected waveform can be displayed on the scope. Student can learn logic '1' and '0' and pulse coding.
Figure 21: Detecting remote control signal. Scope is set to NORMAL with preset trigger level.

Detecting waveform from many sound sources Sound probe can be used to learn much more. Since the sound pressure is everywhere. Students can learn the waveshape from many sources. So many phenomena with sound propagation can be easily learned.
Figure 22: Detecting sound pressure with sound probe.

The prototype can be put into plastic box and have the sticker sheet put on the keypad.

Figure 23: Scope box made with acrylic plastic

Figure 24: PCB inside the box.

Figure 25: Sticker sheet for keypad functions.

Download Workshop Handout Day1 cheap oscilloscope , Workshop Handout Day2 sensor probes, Schematic , Inkscape panel sticker

More information please contact


4 May 2014