Workshop related projects

This page contains projects I have build for my workshop.
 
 Audio signal injector / tracer
 Earth Leakage Measurer
 Lamp ballast
 Shaker table
     A few notes on using PIC's
 UV exposure unit / light box
 
 
Audio signal injector / tracer

This device consists of a sinewave oscillator which can generate a 100Hz, 1kHz or a 10kHz test signal. It has a line output as well as a low level balanced microphone output. The second part of this device is an audio amplifier with speaker.



Earth Leakage Measurer

I have build this device to measure the earth leakage current of an appliance. It measures the current across a low serial resistance in the earth line. A instrumentation amplifier is then used to amplify the signal. An oscilloscope may be connected to the output. This way it is possible to see the waveform of the earth leakage current. The device also has a meter which measures the peak current. It also has a PEAK HOLD mode. This comes in handy in detecting spikes (such as when a PC is switched on.)
 
 


Lamp ballast

I have build this device for two functions. First I needed an overcurrent protection device when I tested low-power mains equipment and secondly it serves as a light source for my microscope.

The device which needs to be protected is plugged into one of the sockets at the right side of the device. The power is switched on and off by the power switch. The LAMP / BALLAST switch selects the operational mode of the device. The other switch is a multifunction switch. If the device is in ballast mode, and the SWITCH is in the BYPASS position the light bulb is bypassed. If the device is in LAMP mode and the switch is in the DIM position, the light bulb is dimmed by placing a rectifier diode in series with it.

A standard incandescent light bulb forms a very effective short-circuit protection device. The bulb is placed in series with the circuit's power line. When the filament is cold, it have a relatively low resistance. As long as the protected circuit does not draw a lot of current, the filament will not heat up. On the other hand, if there is a fault in the circuit and it draws excessive current, the filament will heat up and its resistance will increase. This will limit the current. When the fault in the circuit is restored and the current drop back to normal, the filament will cool down and its resistance will drop, restoring normal operation.

The light bulb I used for the above unit is a 60W bulb. I have measured its resistance when the filament is cold to be 80 ohms. I have calculated that the hot filament resistance must be about 800 ohms (the bulb is a 220V version). The short-circuit current will therefore be 275mA, which is not so high as to damage a circuit that quickly.

I have also used 5W 12V car light bulbs as an effective short-circuit protection in 12V systems. In most cases the worst what will happen with a short-circuit is that the bulb light's up, which can be used as an indication.



Shaker table

I got some parts out of an old printer and decided to build a shaker table which I can use to speed-up the etching of PCBs. Below is a photo of the finished shaker table. The table on top moves sideways.

Below is a top view with the table removed. The off-center spindle which causes the sideways movement of the shaker table can be seen in the center. The mains transformer is to the left, while the motor is to the right. The power electronics board is at the back of the unit. The gears are used to transmit the motor power to the off-center spindle and also to reduce the speed.


The shaker table uses a single phase induction motor. The motor has a main winding as well as a starter winding. The mains voltage is passed through a transformer and then rectified. The filter capacitor consists of two identical capacitors in series. The one side of the motor's winding is connected at the junction of these capacitors to get a voltage midway from the total voltage. The other side of the motor's winding is connected to two MOSFETs. For a schematic of the electronics click  here . Below is a graph showing the waveform send to the motor:


I am using a rectangular waveform to approximate a very crude sinewave.

The motor I am using is a 100V motor which runs at 1200rpm with a 50Hz line frequency. The voltage, Vo, is 100V in my circuit. If I want to send a 100V sinewave at 50Hz to the motor, the total period should be 20ms. A half period is therefore 10ms. Furthermore a 100V sinewave has a rms voltage of 70V. To keep the rms voltage of my waveform the same as a pure sinewave, it need to make the duty cycle of my pulses 70%, which makes the pulse width 7ms.

In practice it means that I will switch on the top MOSFET for 7ms, then I will switch it off and wait 3ms. I will then switch on the bottom MOSFET for 7ms, then I will switch it off and wait 3ms. I will then repeat the whole sequence.

The motor speed is controlled be controlling the frequency. When the frequency is reduced, the rms voltage should also be reduced, otherwise the motor's magnetics may saturate which will cause the motor to overheat.

The start winding is switched on and off by means of a solid state relay. A capacitor in series with the start winding ensures that the correct phase difference is obtained to get a turning magnetic vector in the motor.

The control circuit consists of a PIC12F675 microcontroller. I use the ADC of the PIC to read in the voltage from the speed control knob. There is also a optical sensor detecting motor movement. The PIC reads in the value of the speed control knob. It then use two lookup tables, one for the off-time and one  for the on-time. The PIC's 16-bit timer is used to generate the timing for the pulses to the MOSFETs.

Whenever the signal from the optical sensor stays the same for a certain amount of cycles, the start winding is activated. The start winding will then be on by default for some cycles to get the motor moving. I have had the problem that on low speed the motor starts up, but the moment the start winding switches off, the motor stopped again. I therefore changed the software so that the start winding is permanently on at low speeds. I compensated for the extra motor current by reducing the rms voltage supplied to the motor at low speeds.
 
A few notes on using PIC's:

While using the PIC12F675 I have learnt two very important things about the PIC.

The MCLR should not be left unconnected when 5V is supplied to the Vcc pin. Since this pins must be able to handle a voltage higher that Vcc, it does not have the same effective ESD protection as the other pins. With most PIC's this pin should be connected via a 1k resistor to Vcc (that is if it is not to be connected to any other device). If this pin is not connected and power is applied to Vcc, it will damage the device. The PIC may still be operational, but it may no longer be reprogrammable.

Upon processor reset the GP0 and GP1 pins are configured as analogue pins. This is a problem if these pins should be used as digital pins, as all analogue pins are read back as 0. If any bit operation is performed on the IO port, these bits will be cleared.  To overcome this, the analogue comparator's mode should be changed so that GP0 and GP1 are no longer analogue pins. This also apply for the analogue pins on the PIC16F87x family.
When another analogue input channel is selected for the ADC, the conversion should not be started immediately. If the conversion is started immediately, the ADC's sample and hold capacitor will not have had enough time to charge to the new analogue input's value. The value converted will thus be incorrect. It is best to select the new analogue input, then do something else, and then to start the conversion.
The above notes are also applicable to the PIC16F87x family of microcontrollers and possibly also other.


UV exposure unit / light box

This unit can take PCB sizes of up to A4 size. It can either generate UV light for PCB prototyping, or white light to be used as a light box. It contains 4x 8W UV tubes and 4x 8W white tubes. It has a digital timer which can count up to 10 minutes. Below is a photo of the finished unit:



Below is a photo of the electronics inside the unit. The components at the backside is from left to right the IEC mains input EMI filter (hidden from view), the mains transformer supplying power to the low voltage electronics and the high voltage PCB. The high voltage PCB contains the inductor chokes, high voltage mains rectifier and filter capacitors. The PCB on the left hand side contains the low voltage rectifier and filter capacitors, a 555 circuit, isolation pulse transformers and the power MOSFETs. The 555 circuit generates the square wave which is used to switch the MOSFETs. Just behind the front panel is the microcontroller based timer PCB.



Below is a photo with the internal covers mounted. The holes are for ventilation purposes.



Below is a photo of the unit with the cover fitted:





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