This is a simple DC Electric Screw Driver Motor Controller circuit which I designed for building my own electric screw driver. I want to call it TORQUE. A DC motor is operated in forward and reverse directions using two push buttons. An adjustable current sensor circuit will detect over-current through the motor in forward/reverse operation and halt the operation of the motor automatically when the screw gets tightened enough.
The circuit consists of three main sections - an H-Bridge, a current sensor and a bi-stable latch. The H-Bridge uses BD140 and BD139 complimentary power transistors which have a max. collector current of 1.5A. That means you can connect a DC motor which draws more than 1A. The diodes D1-D4 are used to prevent the back-emf from the motor to the transistor. Use 1N4148 for low power motors and 1N4007 or 1N5819 for high-power motors. The Q5 and Q6 will trigger the H-Bridge using a very small current. Q5 controls forward and Q6 controls the reverse operations. Note that closing the two push buttons at the same time will short circuit the H-Bridge and heat damage the transistors.
The over-current sensor circuit was taken from the website of Roman Black. The current sensor consists of three current sensing resistors - R5, R6 and R7 in the circuit. The effective resistance of these resistors decides the max. current you can detect. The less the resistance the more current you can detect. I used 0.3 Ohm here. The Q7 will drive a digital output to LOW if an over current is detected. The output of Q7 is given to a bi-stable latch. The sensitivity can be adjusted using VR1. Increase the value of C1 if you have problems with motor start-up current. The purpose of Q8 is to enable the over-current sensor only for the forward operation.
The bi-stable latch consists of an NE555 timer configured as a bi-stable multivibrator and a switch Q9. The over-current sensor and bi-stable latch will only work if the forward switch is closed, making the circuit more efficient. As long as no over-current is detected by the current sensor, the output of Q7 will be HIGH. Therefore the output pin 3 of 555 will be LOW which turns the transistor Q9 on and the motor runs forward. When an over-current is detected, the output of Q7 goes LOW and it will trigger the latch. This makes the output pin 3 of the 555 goes HIGH which will then turn off the transistor Q9 and cease the current to the base resistor of Q6 and motor stops. This condition stays latched until you release the button to reset the 555.
The DC motor for the screw driver was salvaged from an old VCR. It has a worm gear built-in which is powerful enough for medium screw driving. And that is also the reason why I started building a screw driver with it in the first place. As you can see in the images, I used a China made removable-bit screw driver to make the driving mechanism along with the motor. The bit holder has magnet inside where the bits will stick to. You can see in the below image how I connected the bit holder to the motor.
A ball bearing is inserted into the bit-holder and it is enclosed and glued in a PVC pipe of diameter__. The bearing is of type 608DSD28.
The metallic locking pins inserted into the plastic base of the bit-holder will receive the torque from the motor's gear system. The motor and the bit-holder mechanism are attached to a wood base to make contact. Here, the locking pins can move a bit freely inside the gaps of the gear wheel, which will reduce the motor starting current sometimes.
The motor along with the driving mechanism is inserted into an another PVC pipe which acts as the screw driver hand. Two push buttons are fixed onto the pipe for forward and reverse operation with thumb. To the bottom of the pipe, I added a USB female port in order to connect it to the circuit box.
Five lines were needed to connect the screw driver mechanism to the circuit box - four USB terminals and the body GND.
I first built a prototype of the circuit in a breadboard and tested it. Then built the whole circuit in a perforated board. I used thin wires and enameled copper wires to make the interconnections. Care should be taken to avoid any short circuits. Solder the small components first and then to the larger ones like electrolytic capacitors. Check the connections again and again.
I used a speaker box to install the circuit board and transformer for the system. Built a panel with wood sheet with all the necessary controls. It uses 12V DC from a low power transformer which can provide up to 600mA current maximum which is enough for the motor. A linear 47K pot will control the sensitivity of the screw driver. If the pot is set to a low value (to the left), the operation of the motor will be halted with a little force on the bit. That means the screw will be tightened only lightly and vice versa. The torque adjustment switch is used to change the value of the electrolytic capacitor, to give more or less time for the current sensing circuit to act. The polarity switch will allow to reverse the forward & reverse operation. If it is set to reverse polarity, the current sensing and auto-stop action will be applied the reverse operation. I've seen it in some foreign devices.
Everything was completed and the screw driver works fine though I'm not so happy with the aesthetic perfection of the build. May be I should rebuild this in future. Mm.. But still I got one more home made thing to put on top of my table.