Let's Make Robots!

Johnny 555 (isn't quite alive.)

Look for light (supposedly)

(Note:if you're seeing a helicopter and compressed air rocket, that's not right.)

The other night, Towlieboy78 said he was having trouble with his Photovore in the shoutbox and pointed us to http://letsmakerobots.com/node/33368. I took a look at the schematic:

The first thing I noticed was that there aren't any caps in the circuit, which the 555 generally needs to regulate itself (it is after all the most basic timer chip on the bench.) So suffice it to say, I was skeptical. Second off, I couldn't really see how you'd get a big enough controlled voltage difference on either side of the trigger to cause a diff drive to pull one way or the other.

In any case after some admonishments from other LMRtians for my doubt, I decided to put it to the test. I built it on my trusty MicroMagician chassis, chose a known good IC and two well-balanced LDRs and even added a 4pack of AAs (the MicroMagician comes with 3pack holder.) The result?



As you can clearly see in the video the damned thing is as blind as Stevie Wonder in a cave during a power failure. It doesn't stick or react to ambient light umbras or turn into bright light sources(my super flashlight, for example.) Neither does it turn away from it (which would make it a photophobe) or give any kind of reaction at all. It generally kind of ambles around like a damned boll weevil. If it's reacting to anything but noise in the circuit, it's not an output which is dependent on any inputs.

Not to poop on Chowmix's build, but I didn't see his video until after I'd built my version, and his doesn't seem to react to light either.

I know I sometimes am a bit harsh about Beambots, but I'm willing to give them a try for the fun of it. However, builds are only fun when they work, and I've never had a classic Beambot design function the way it's advertised. From 74xx245 bi-cores to 6 transistor h-bridges, I'm always disappointed at the functionality and complexity compared to simple micro controller circuits.

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My robot worked fine. Not as good as a binary algorithm on an Arduino, but it was a much simpler design. In the video, the robot is following the flash on my iPhone as I recorded it with the same device. 

Nope, you've done something wrong.

The two LDR's form a voltage divider and are cnnected to the trigger (pin 2) and threshold (pin6). When the voltage between the LDR's is less than 1/3 of Vcc then Pin 3 will go high. When the voltage is above 2/3 of Vcc pin 3 will go low.

The way the schematic is drawn there should only be one motor on at a time. The fact you have the robot driving forward tells me you have either wired it wrong or are using a CMOS version of the 555 chip.

The TTL version of the chip can sink or source 200mA on pin 3.
The CMOS version can sink 100mA or source 10mA!!

Even if you are using a TTL version, your motors cannot draw more than 200mA. If they do then pin 3 cannot cope and the two motors are simply wired in series across your battery. This would explain your robot just driving forward.

If you are using a TTL 555 then you can either reduce the voltage to 4.5V to help reduce the motor current and / or wire a second 555 in parallel (stack the chips) to double pin 3's sink /source current to 400mA.



I trust your take on things here OddBot.  It just doesn't interest me enough to pursue it further, I guess.  I so rarely feel good enough to put time in on the bench anymore that I'm probably not going to investigate this type of low-functioning toy a lot.

What's the chassis you used?

Now I see they've changed it to "2 Wheel Drive Simple Robot Base Kit"

A crusher of dreams you are.