Light Sensing Bristlebot

This "level up" from your basic brainless bristlebot has the capacity to seek out or "run away" from light sources. Its great fun to lead through a maze with a lighter or torch - like a moth to the flame. Install the light sensors the other way around and it takes on cockroach characteristics, moving away from the light.

This design also employs a capacitor to power the motor, allowing for endless recharges.

A great "next step up" project for fine tuning your soldering techniques. In my experinece total beginners (11 year olds with no previous soldering experience) can manage these but need a little support and guidance here and there.

See it in action

Tools:

• Soldering iron with a reasonably fine point
• Small wire cutters
• Large wire cutters
• Solder

Materials:

• 1 Cylindrical supercapacitor. Capacitance 1F - 5F voltage 2.7V - 3.6V. 2
• (see extras for notes on capacitors)
• Darlington phototransistors (light sensors)
• 2 Cellphone buzzer motors, 3V.
• A toothbrush (an old one is fine)
• A pair of batteries to charge your capacitor

Bits:

• Small length of narrow heat shrink tubing.
• A small offcut of wire to connect the terminals of the batteries
• A piece of tape

This exercise has 3 parts – The bot unit, the brush, and the charger.

1. Identify the negative and positive leads of your capacitor – there is usually a stripe along the negative side, and the positive lead will be longer. These will look like antennae when your buzzbot is assembled, but they are actually used for charging the capacitor, to power the motors.
2. Trim the longer lead so that both are the same length. This is mainly cosmetic, but does make charging a little easier.
3. Thread the piece of heat shrink (really only 2 mm!) onto the positive lead of the capacitor and shrink it in place. It doesn’t matter if it’s still a bit loose, it will be fixed in place as we go ahead.

1. Take a look at your light sensors- these are directional so you need to identify the positive and negative leads as well as the front and back. Hint - The side with the bump on it is the front (needs to be facing forward in your finished bot). The negative lead is the one with the big ‘block’ within the sensor.

1. Bend the negative terminal of one light sensor at about 1cm (the further down the longer your “eyestalks will be) – see picture Trim the negative lead on the other light sensor at about the same position as the bend in the first.

2. You now need to solder the two negatives together as shown. 1. Prep the leads with solder (tin them with enough solder to make your join). 2. Hold the pieces in place and solder together (helping hands or strategic blu-tak will help here).

3. Line up your sensors on the capacitor as shown – the negative lead of the sensor on the right needs to meet the negative lead of the capacitor (we will attach these in a later step), while these are insulated from the positive lead of the capacitor by the heatshrink.

4. You can bend the positive leads of the sensors back at 90 degrees so they will lie along the side of the capacitor.

1. Tin (add a little solder to) the ends of both leads on both motors.
2. Add a dot of solder to the positive lead on the capacitor – just above the heatshrink.
3. Now attach both red motor leads to that dot of solder on the positive capacitor lead – twisting the wire ends together first will make this easier.

1. Add a dot of solder to each of the positive terminals of the light sensors just after the bend.
   Attach the blue lead of one motor to each.

2. Line up your light sensors on the capacitor again – the negative lead of the sensors should contact the negative lead on the capacitor.

1. Organise your components so that each motor is on the opposite side of the capacitor to its sensor (blue lead attached to sensor on opposite side), this ensures your bot will follow a light instead of running away from it.
2. Fix in place ready to solder (use helping hands or blu tak down the side of the capacitor).
3. Solder the sensor/capacitor connection.

4. At this stage you might want to trim the positive capacitor leads or leave them as is – this is purely cosmetic and up to you.

5. Splay “antennae” slightly in order to reach / fit charging station – see charging station instructions.

6. When you come to assemble your bot ensure the motors and corresponding light sensors are on opposite sides in order to ensure correct turning patterns (turning towards light sources). Attach motors firmly (to the capacitor or side of the brush) to ensure maximum vibration transmission to bristles.

You can use any small brush for this, but we think toothbrushes work best for a bot of this scale.

Consider:

1. Wider brushes will be more stable.
2. Bigger (heavier) brushes will be slower.
3. Used toothbrush bristles are nicely splayed so tend to be stable.

The key thing is to have angled bristles. To do this we modify our toothbrushes using hot and cold water.

1. Dunk the brush into a cup of boiling (recently boiled) water for about 30’s (this is not a precise measure, just enough to soften the plastic.
2. Run the cold tap in the sink through the next step.
3. Smoosh the brush on the bottom of the sink – sliding the brush forward will ensure the bristles angle backwards – try to keep it level so the bristles are evenly smooshed.
4. The bristles should be quite smooshed at this point – don’t worry, they will bounce back.
5. Trim the handle off the toothbrush.

6. Wait for the bristles to dry completely (they don’t work very well when they’re wet).

7. Attach you bot, charge and go!

In assembling your bot ensure the motors and corresponding light sensors are on opposite sides in order to ensure correct turning patterns (turning towards light sources).

Attach motors to the toothbrush firmly to ensure maximum vibration transmission to bristles.

Try different positioning of the unit and motors to see what works best (we use blu-tak for attachment to allow modification).

1. You need 3V to charge these capacitors.
2. We used 2 AA’s (bigger batteries will get you more charges).
3. Connect one set of terminals (+ve to –ve) using whatever conductor you have about (wire offcuts are ideal). You can solder or stick.

Soldering – you really need to heat the battery terminal in order to ensure the solder will stick (or it will just break away).

Sticking – simply taping a wire across the terminals at one end (+ve to -ve) will do the job, but tends to shake loose so you need to check/ hold it firmly in place when charging.

1. Tape the batteries together.
2. Charge – complete the circuit using the capacitor, touching the negative to negative and positive to positive – the capacitor will charge fully in a few seconds.

Capacitors

1. 1F capacitors work great, but larger ones will run the bot for longer.
2. Capacitors larger than 5F are physically too large at the present time to fit comfortably on a toothbrush head.
3. DON’T use a capacitor with a higher voltage rating than 3.6V - high voltage capacitors can cause sparking and burns. We find that 2.7V, 3F capacitors are the best.

Buzzy motors

1. We use 10mm disc or button-type cellphone buzzer motors.
2. Make sure to use buzzer motors, not the linear actuator buzzers used in high-end smartphones, which look the same but are more expensive and need a computer chip to drive them.
3. There are other kinds of cellphone buzzer motors that will work, but we find that the 10mm disc motors work best.

Toothbrushes

1. Wider toothbrushes work better.
2. Most toothbrushes are too narrow, so your bot will fall over easily.
3. Manky old used toothbrushes with the bristles splayed and curled often work well, and can give your buzzbot a unique personality!

Known design flaw

1. The connection between the capacitor and light sensors is a weak point and does tend to rattle free eventually (with all that buzzing) - but is an easy mend. We do have a better solution for this - coming soon.