Saturday, December 3, 2016

Potato Gun Review

We made progress, but we had some bugs. I found it useful to myself to write down what I had done and plan for what to do. I figured I'd share it since yes and no is not that interesting.

We were able to trip the circuit, and use the Arduino code to interpret it ( The speed calculation is built in on line 53. The inputs from the receiving circuits should be A1 and A2.

We used some parts from a Parallax sumo-bot kit as pictured in
and (
and pg. 45 - 49 of

I seem to recall, we may have had some occasional issues with sensitivity, so it was a challenge to trip them at times.

We never got the final setup with the pvc pipe and reflectors to work.
I do not think they ever were tripped.
( This appears to be putting the IR transmitter and receiver side by side with communication by bouncing off the reflector (compare photogate1.JPG to
It looks like we tried the sumo-bot parts. I may have been worried about transmission loss at the time and also how much a potato may shadow the signal by passing through the parachute shaped gates. The transmitter and receiver may need to face each other within the transmission and reception angle.

I may want to adjust the sensitivity of the receiver with an op-amp and the power of the transmitter with a potentiometer or pwm setup.

I'd like to go back and try it again with the infrared LED emitter and detector package from RadioShack
A skilled observer who acted as an electrician / EE consultant thought it was simpler than the sumo-bot circuit. Perhaps it will befriendlier to my partner and I. We got it to light, and our cell phone(?) cameras were able to pick up the emission
I think we should amplify the received signal to the two receiving diodes with an op-amp circuit and perhaps adjust the brightness of the two transmitting diodes with pwm or a simple potentiometer. The potentiometer setup already worked with the Sparkfun Redboard
( /11800522_10103322872952857_7659406262285231569_n.jpg)
which is an Ardunio knockoff.  I'm treating the receiving LED with the op-amp as a photodiode ( It might be worthwhile to apply the op-amp circuit and pwm / potentiometer to the sumo-bot parts too.

For the op-amp circuit and pwm/potentiometer circuits, I constructed some circuit diagrams in Fritzing (file names in parentheses) that were based off ones I found on the web.

I went online to find some op-amp circuits. I ended up going with this one:

I also found some led-dimmer circuits including or excluding a power rectifier and transistor (in order of more parts to less):

I constructed the circuit for the potentiometer in the video
(potlednove26_bb.pdf) It looks like I already built it with the
Sparkfun Redboard as the battery.

I think we can either use the potentiometer circuit or the pwm circuit
to adjust the brightness of the LED. The potentiometer circuit is a lot simpler. I'm not certain of its limitations. It is a variable resistor. It is all analog, and I recall from the web that it may only be good for a small number of LEDs (although I don't know why this is true). The pwm circuit adds a 555 timer, a few capacitors, resistors, diodes, and perhaps a power rectifier and transistor to the potentiometer. The 555 timer produces a square wave so I call it

I e-mailed myself with some parts (mostly from that matched the circuit diagrams I found at the links above. I need to construct a purchase order for myself, and make this thinking more formal, mainly so when I go back to Oklahoma I have something to do with my project partner.

My partner was also worried about the lack of a rigorous method for sizing the potato gun. The potato guns had previously been sized empirically and with inspiration from sources such as Burt Latke's studies. I built a spreadsheet to scale based on size ratios if I recall correctly. This is described in a quote from :

"Performance in terms of velocity of the potato gun was found to change as various ratios in the geometry changed. Performance was judged by the damage that the gun did to targets.The most damage was done when the chamber diameter was at least twice the barrel diameter with a ratio of length of chamber to diameter of chamber between 3 and 6. It was less apparent, due to limited experimentation, but the chamber volume was usually kept at 1.2 times the barrel volume. This was inspired by a website based on Burnt Latke's studies

We had no mechanism to measure the exact amount of fuel we put in other than our own sense of time when spraying the propellant. So, even if we got the photogate to work we may not measure a consistent speed due to variance in the propellant amount. If we fix the propellant amount,we might find the maximum speed for a particular potato gun.

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