Servo - January 2007

reach. The simplest solution to stopping a robot in an emergency is to install a wireless kill switch that disables motor drive circuitry when activated.

An inexpensive kill switch system is available from All Electronics Corp., in the form of a keychain remote control shown in Figure 5. The $19 system — which includes a pair of 27 MHz wireless key fobs — is designed to supply 12V to a remote device. However, the main board can be reconfigured so that the contacts of the mechanical relay are available for direct or indirect control of the robot motor system. I purchased several of these units and modified the solder pads on the boards and remotes so that any fob can control any and all robots with a kill switch – an especially handy feature when working with multiple, fast-moving robots. Line-of-sight range is about 100 feet, and about half of that indoors.

A major limitation of using a kill switch as a remote safety device is that it requires you to recognize something is wrong and then press the “off” button to deactivate a robot. An alternative approach is to use a dead man switch, which is roughly equivalent to a watchdog timer in that an active signal is required to keep the robot energized. If the onboard receiver doesn’t detect a signal because you released a momentary-contact switch on the remote, the receiver is out of range, the battery is dead, or any number of other reasons, the robot can be configured to shut down.

Although it’s hard to beat the form factor of the fob shown in Figure 5, the Parallax 433 MHz transmitter and matched receiver can be used to create a relatively compact dead man switch. As with most Parallax devices, setup is quick and painless. Other than supplying each device with 5V at 5 mA, installation is a simple matter of interpreting the analog received signal strength value with an onboard processor or using it to directly control a solid-state relay. Line-of-sight range is about 500 feet.

threat to eyes, ears, and extremities. The simple act of trimming a solder joint with a pair of diagonal cutters can launch a wire with enough velocity to embed it in an eye.

Unfortunately, not all construction safety techniques can be gleaned from common sense. If you’re unsure how to do something, then get help. Someone skilled in cutting and bending aluminum sheet can show you safe techniques that could take years to master. Given this caveat, some basics that apply to all robot construction projects include:

area with a fire extinguisher rated for class A (common combustibles), B (electrical), and C (flammable liquids) fires. An ABC extinguisher based on Halotron is ideal for robotics work because the hydrochlorofluorocarbon is an electrical insulator that doesn’t leave a residue.

• Ventilate your work area when soldering or using adhesives. Avoid breathing flux vapors, which can cause permanent respiratory problems.

• Wear eye protection when using power tools and when cutting wire, plastic, or metal with hand tools.

• Use leadless solder and components, and wash your hands after handling solder. Children are especially susceptible to lead poisoning.

• Wear ear protection when working with power tools to prevent hearing loss. The high-pitched whine of a Dremel can be muffled with in-ear plugs available at most hardware stores. In-ear plugs, while not as comfortable as over-ear protection, offer superior sound reduction and protection from potential hearing loss.

• Use multi-strand, insulated wire for connections that require flexibility, and insulate exposed terminals and wires with shrink-wrap tubing or electrical tape. Solid wire is fine for use with plastic prototyping boards, but more flexible stranded wire is generally a better choice for interconnecting batteries, sensors, power supplies, and processors – especially when there may be relative movement between components.

• Wear short sleeves when working with power tools. Similarly, avoid loose jewelry and anything else that might become caught in a power tool.

• Use clamps with a drill press to avoid a trip to the emergency room to reattach your fingers. Clamps are especially important when step bits are used, because they are more likely to bind than straight bits (See Figure 6).

• Use sealed, leak-proof batteries whenever possible. In addition, don’t leave batteries in your robots for extended periods – even if they are leak-proof.