ECE 693 Homework 1, Problem 10

Digital Thermometer

Work in progress...

I built this circuit on the breadboard last fall and wrote some code for a micro controller to convert the analog temperature readings into a digital format on a LED screen, but the code for the LED screen is tedious and my time precious. The circuit works and the temperature reading are accurate over a range from freezing to boiling in degrees Fahrenheit. Once I get some free time, I'll put pictures up of the construction process for assembling the components onto a stripboard (Veroboard) with details explaining how the components work in conjunction with one another.

The two potentiometer (rheostats) allow you to calibrate the circuit so that the temperature readings are accurate. When the circuit is constructed on the stripboard, I think I will use smaller, board mountable, potentiometers so that as the components degrade, I can still adjust the performance instead of just using a single resistance in its place.

The clear tube with the black end is actually a straw with a transistor inside. A portion of the transistor is sticking out the end of the straw with the legs sealed from the outside, so that I could dip it into extremes of boiling water and ice water to aid in calibration. I'm well aware that you can buy a cheap thermometer, but I thought it would be cool to explore the aspects of the transistor. The theory is that if the current is kept constant through the transistor, then the voltage will vary linearly with the temperature. Thus, it is just a matter of reading the voltage and applying a scaling quantity to equate it to the temperature.

I know the circuit on the board is messy, but it was put together hastily just to ensure that the design worked.

Mousey The Junkbot

Jump to - Step 1. HOW THE ROBOT WORKS
Jump to - Step 2. BREADBOARD THE CIRCUIT
Jump to - Step 3. PREPARE THE MOUSE CASE
Jump to - Step 4. CREATE THE EYES
Jump to - Step 5. BATTERY AND MOTOR PLACEMENT
Jump to - Step 6. MAKE THE TIRES
Jump to - Step 7. ADD THE BUMP SWITCH
Jump to - Step 8. INSTALL THE POWER SWITCH

THE LIGHT SEEKING ROBOT

The goal of this project is to build a cheap and simple light-seeking robot. One way this can be accomplished is by scavenging parts from obsolete electronics that may be lying around the house. To begin with, we will start by obtaining an old analog “ball” mouse that will supply us with a body as well as two light sensors and a SPST bump switch. Some common electronic devices that will negate the need to buy some components are VCRs which contain small motors, answering machines for Op-Amps, and analog modems for DPDT relays just to a few. All the parts listed below can also be found at your local Radio Shack or other electronics retailer if you prefer not to destroy your electronics. When acquiring these electrical components, try to get the smaller parts, since the objective is to have everything fit inside the mouse case.

Tools:	Parts:
(A) Analog "Ball" Mouse Case	Dremel with drill and cutting disks
2 Light Sensor Phototransistors	Soldering iron with solder
Single-Pole Single-Throw (SPST) touch switch	Desoldering Bulb
(B) Single-Pole Single-Throw (SPST) toggle switch	Wire Strippers/Cutters
(C) Double-Pole Double-Throw (DPDT) relay	Needlenose Pliers
(D) Audio Operational Amplifier (Op-Amp)	Digital Multimeter (DMM)
(E) 2 Small DC motors	Breadboard
(F) NPN Transistor	Superglue/Hot Glue
(G) 1K Resistor	Phillips Screwdriver
(H) 10K Resistor	Hobby Knife
(I) 100µF Capacitor	Ruler
(J) Light-Emitting Diode (LED)	Safety Glasses
(K) 9V Battery
(L) 9V Battery Snap-on Connector
(M) 22-24 gauge wire (stranded and solid-core)

1. HOW THE ROBOT WORKS

The op-amp is the brain of the robot and acts as a comparator between the emitters to determine which motor should receive more voltage. When more light is received by one emitter, then the op-amp provides more power to the opposite motor so that the robot always redirects it self to the lightest location in the room. The LED and 1K resistor create an indicator to acknowledge that the power is on. The bump switch, transistor, 100µF capacitor, 10K resistor, and relay allow the robot to back away from any obstruction. When the bump switch is triggered, the relay will switch the polarity to the motors and the capacitor will dump its stored energy to the motors so that they reverse direction. The length of time that the robot is in reverse is determined by the time constant of the resistor and capacitor pair, thus an increase/decrease of either will also increase/decrease the reverse time. To simplify the robot even further, or if the mouse case cannot support all the components, the reverse circuity can be removed.

2. BREADBOARD THE CIRCUIT

Before you begin to assemble the circuit inside the mouse case, it is recommended to first use a breadboard to easily test the circuit without soldering to ensure that everything works like it should. In the schematic specific part numbers are used, such as the LM386 for the op-amp and the 2N3904 for the transistor. Other op-amps and NPN transistors will work, but I have found that these are the most common.

After you have hooked everything up and connected the battery, the motors should spin and by placing your hand over an eye should cause the corresponding motor to increase in speed. You can also use a flash light and shine it on an emitter to cause the opposite motor to increase in speed. If the opposite occurs, the eyes are wired backwards, then the mouse will seek the dark.

NOTE: The circuit diagram above depicts the connection for a 5V relay. I could only find the larger 12V relays at Radio Shack. These will work, but they are more cumbersome to work with and the connection is different. To connect the 12V relay correctly, hook up the left and right motor to pins 11 and 6 respectively and connect pins 9 and 4 together as well as pins 8 and 13. Also not shown in the above circuit is the fact that the left eye connects to pin 3 and the right eye connects to pin 2.

3. PREPARE THE MOUSE CASE

The circuitry that will be put inside the mouse will be free-formed, meaning that the components will be soldered together instead of soldered onto a board. First you will need to remove all the mechanics and electronics inside the case. Next, using a dremel with a cutting disk attached, start clearing out the inside of the mouse case to make room for the circuitry by cutting away all the partitions and plastic mounts except for the screw posts and mounts securing the buttons.

4. CREATE THE “EYES”

For the “eyes” of the robot, we will be using two IR emitters scavenged from the inside of the mouse case. The emitters will be made of clear plastic and located near the decoder wheels. You will need to desolder the components from the printed circuit board (PCB). Once the “eyes” have been removed, you will need to determine which pin is the anode (+) and cathode (-). Using a digital multimeter (DMM) in diode check mode or as a voltmeter, touch the probes to the pins. When you get a reading around 1V then your red probe will indicate the position of the anode.

Next, you need to add some eye stalks to the emitters so that they can be easily adjusted. Cut four strands of 6-7 inch solid-core wire and solder the wire to the pins. Solid-core wire is used on the stalks so that the eyes stay in a fixed position. The rest of the circuit should be connected with the stranded wire because the wire is more flexible.

The two phototransistors need to be hooked up backwards. It is recommended that once you discover the anode by getting a positive voltage with the DMM, that you solder a black wire to the pin that was touched to the red lead on the multimeter and a red wire to the other pin. The diagram depicts the connection for this technique by having the red wire (cathode) as positive. The reason for connecting the "eyes" backwards is because when the diodes are reverse biased, the change in current flow is noticeable when light is detected.

5. BATTERY AND MOTOR PLACEMENT

Forethought into the the placement of the motors and batteries is essential since these parts take up the most space and are the heaviest. Thus, when installing the motors, they need to be positioned to balance the weight and set at identical angles so that they support the mouse case. They should also be perpendicular to the center line of the mouse body so that the robot will travel in a straight line. After holes for the motors have been cut and the motors set inside, ensure that the motors are positioned properly and secure them with hot glue or epoxy. I find it is easier secure the motors at the proper angle by cutting holes along the corner on the bottom of the mouse and in such a way that the motor will wedge inside.

6. MAKE THE TIRES

If you are using motors with a gear on the shaft, then tires can be formed by using superglue to secure rubber bands to the gears. You must ensure that the same length is used on both gears or the robot will have a tendency to travel in circles. If the motors you are using are just shafts, then I find that Hot Wheel cars or the guide rollers in cassette tapes make great hubs that can be adhered to the end of the shafts.

7. ADD THE BUMP SWITCH

The bump switch is also a component found on the printed circuit board inside the mouse case. This switch will serve as the whiskers for the robot so that if it bumps into anything, the switch will trigger the relay causing the robot to back up for a period of time. The placement of the switch can be modified for whatever is more aesthetically pleasing. I like orienting the switch so that the wires go through the hole where the mouse cable use to be, which if done properly also means no glue is needed to secure it. To make the switch more sensitive, add a strip of an old credit card or piece of cardboard to the button with a little glue.

8. INSTALL THE POWER SWITCH

The toggle switch will be installed in the back like a tail and will provide a convenient way to turn power on and off. With the dremel, drill a hole in the case big enough for the switch and secure the switch to the case with the nut and washer. If the mouse case has a screw post in the back, try to avoid obstructing it by positioning the toggle switch farther away. If the toggle switch becomes an obstruction when trying to put the mouse case back together, you can cut out the post and use super glue to fasten the mouse case.

Construction in Progress Pictures