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Improved 240 Reversing PhotoVore Circuit from
Behavior of Circuit: Wilf explains the circuit best on the Solarbotics web site. Here is his description.
This is a revised reversing photovore schematic showing R2 and R3. The values of R2 and R3 are experimental and are related to the "resistance" of the photosensitive devices used in the photo bridge.
R2 should be roughly equal to the minimum resistance of the the LDR, etc in bright light. If necesary these sensors should be shrouded or masked so that the current through them is not too high in bright light to avoid power losses. R2 together with C2 limits the maximum frequency of the monocore and motor drivers when the light is bright and the sensors are equally lit. That further avoids power losses as switching losses increase with frequency and motors don't respond very well to high switching frequencies (quivering).
R3 should be equal to the maximum resistance of the sensors in low light. R3 together with C2 sets the minimum frequency of the waggle even in the complete dark which is more interesting than twirling endlessly in a circle.
As I mentioned the values are experimental and should be adjusted for most interesting range of behaviour.
I also show alternative light sensors including 3 lead phototransistors used as photodiodes (AFAIK it is the collector base junction that acts as a reverse biased photodiode. This adapted photodiode is not as sensitive as large area types so C2 may need to be reduced to 0.01uF while the value of R2 and R3 can be increased by a factor of 10. Some LEDs can be used as photodiodes although these are less sensitive still.
Two leaded phototransistors can also be used but may require extra shielding to reduce light current in the bridge to acceptable levels. Note that here, sensitivity is less of a factor than dynamic range.
Construction Notes: Parts include: 74AC240, 2-motors, 2-photodiodes, 2-.22uF capacitors, 10k resistor, 1M resistor, 10M resistor, and switch. You could substitute a feeler of some sort for the switch.
As a side note here, I originally built the circuit up on a socket then put the ‘AC240 in it. However, make sure you don’t have crappy sockets. Mine are fine for circuit boards, but not so good to build on. After I had it all put together and was testing it, I found way too many broken solder joints to the pins on the socket. The metal on the pins just delaminated off. Using the word ‘pin’ is way too generous. They were just a little flat piece of metal.
As you can see from the pictures, I built the photodiode network and reverser as a module. I used a jeweler’s file to cut a groove into the bottom of a Lego and routed the photodiode network’s wiring under the Lego. I am not exactly sure how I am going to mount the reverser yet. More to follow.
I got the motors mounted, engine installed and the reverser switch mounted. If you look closely at the front view, the motor doesn't quite hit the wheels. It needs another layer. I also started looking at the reverser and it doesn't look like it will work effiently. I have a new design in mind. The bot runs well on 5V. I will probably use a 9V battery and a 5V regulator. I may actually do a mini-project on the 5V power supply since I like to use it so much. More to come.
I reworked the reverser switch. I wasn't happy with the way it would engage. I used two switches and made an analog OR. Additionally, I tweaked the mount point for the switch as well. I replaced the original photodiodes. Be sure to test each one to make sure they have the same output (saves you trouble later). The bumper is thin styrene, heated then curved and attached with hot melt glue. I added a layer of rubber to the drive axles. Now they engage the tires fully. Another first for me was a magnetic battery holder. Three rare earth magnets hold the 9v battery really tight. I put a 78L05 (click for the datasheet) input to the switch to turn it on. You really need to make sure you cut power BEFORE the 78L05, otherwise, it will still drain the battery.
This little bot is pretty quick. It tends to circle around till it finds a bright light, then zeros in on it. If the bumper is hit, it does a 180 and takes off again. Pretty impressive. Be sure to check out the BEAM Gallery for some larger pictures!