DIY Electronics

Arduino tachometer

For this project I used a small DC motor. A sandpaper nail file is hot-glued to its shaft as a makeshift propeller. There are three distinct circuits to this project. The first one is to control the speed of the motor. A ne555 timer is used and PWM (pulse width modulation) for that purpose. PWM works like a switch, turning the motor on and off very quickly. The second circuit is an IR sensor. Every time the propeller passes the IR-LED and photodiode a signal is generated and the output goes high. The third circuit comprises of an ATmega328p chip and an OLED 128x64 display. It takes the signal from the IR circuit as input and calculates the rotation speed and shows the results on the display.
Click here to see the video.

NE555 Pulse Width Modulation

This circuit controls the speed of the motor by turning the potentiometer (P1). When the power is turned on the voltage on pin 2 and 6 will be below 1/3 of the 5 volt, because of the discharged capacitor (C1). This results in pin 3 going high. Very quickly the capacitor gets charged and the voltage on pin 2 and 6 reaches 2/3 of the voltage and this makes pin 3 to go low and turn on the discharge pin 7. The open pin 7 discharges the capacitor, dropping the voltage below 1/3 again. Pin 3 shows a square wave output. The percentage of the power turned on in relation to the complete cycle (period on + period off ) is called duty cycle. The duty cycle is controlled by the pot (P1).

When the resistance on both fixed ends (behind the diodes D1 & D2) are equal (the wiper is in the middle) the duty cycle is about 50%. The frequency should stay the same no matter the duty cycle. It's around 282 Hertz. Good explanation is found here!

IR sensor

IR sensor circuit consist of an Infrared emitting LED and a photodiode. The IR Led is turned on all the time. The light can be seen using the camera of a smart phone. The photodiode resistance changes according to the amount of IR light falling on it, therefor the voltage drop across it also changes and by using a voltage comparator (like LM358) the output changes accordingly. A photodiode has a P-N junction and is reversed biased. The cathode (shorter lead) should be attached to the supply voltage and the anode (longer lead) towards ground. The LM358 is an op-amp and it turns the output high when the non-inverting terminal (+) has a higher voltage than the inverting terminal (-).
In my project I used a micropic 12F675 as comparator and it is programmed to turn GP2 (pin 5) high when input pin GP0 (pin 7) has a higher voltage than GP1 (pin 6). The potentiometer is used for fine tuning. When the propeller of the motor passes by it reflects the IR, the photodiode starts to conduct and the comparator turns the output high. The red LED is an indicator of that event. The output signal is input for the third circuit.

ATmega328p

The ATmega328p chip receives the input signal on pin 2 (physical pin 4). Pin 2 is one of the two interrupt pins. In the Arduino sketch an interrupt routine is called on the rising edge (when pin 2 goes from low to high). A counter is incremented. After 3 seconds the interrupt routine is halted (detached) and the counter is used for the calculation of hertz and rpm. The result is written to the oled 128x64 display and the interrupt routine is restarted (attached). A variable used in an interrupt routine must be declared as volatile and do not do any calculations with this variable. It messes up the routine. Just copy it into another variable.
>>> Arduino code <<<

Brushless motor with IR sensor

The fidget spinner has a magnet in each of its 3 arms. The electromagnet is made out of a plastic spindle from a sewing machine and the wire used to wind onto the spindle is coated copper with an enamel coating that must be removed from the ends to make an electrical connection. The wire is 34 AWG and a drilling machine with variable speed is used to wind the coil. Turning the electromagnet on and off is done by an IR sensor circuit, the same circuit as for the tachometer. Rotation direction depends on the positioning of the IR-pair as demonstrated in the video. To power the coil 9 volt is applied, but the IR-sensor circuit has a maximum of 5 volt. An LM7805 voltage regulator brings the 9 volt down to 5. There are 4 bypass capacitors, 2 on the input and 2 on the output. Their function is to smoothen out any spikes in the voltage.
Click here to see the video.

Electromagnet       IR sensor

Brushless motor with reed switch

The electromagnet is turned on when a magnet is near the reed switch. The contacts inside the switch close the circuit. The reed switch is also used as a clock signal for the decade counter 4017. Only 5 of the 10 possible LEDs are used. The 22 uF capacitor is necessary to smoothen the voltage spikes or the 4017 IC goes bonkers. The rotation speed is about 40 Hz using a strobe light app.

Click here to see the video.

Simon says

"Simon says" is a memory game with 4 buttons, 4 LEDs and a speaker. Follow the pattern of lights and sounds and repeat the same combination. It starts with one light (+sound), then 2 and so on, until it reaches 10 and the game is over. As a player you have to memorize the series of lights and sounds and than reproduce it by pushing the buttons in the same order. The challenge was to write the Arduino sketch for this game. Click here to see the video.