Toy chopper teardown reveals some basic but unusual design guidelines

Bill Schweber, Planet Analog

October 29, 2008

Bill Schweber, Planet AnalogOctober 29, 2008

Embedded Systems Conference, Boston, Mass.—At a live teardown at the Embedded Systems Conference here, Rich Nass, Editorial Director of Techinsights' Embedded Systems Design and John Day, Microchip Technology Field Applications Engineer, took apart an Estes Micro-Tiger remote control helicopter ( Unsophisticated, you think? Not at all: in fact, it's a marvel of clever engineering and safety circuitry, selling for under $30. Day noted "products like this have a very high ratio of function to electronics."

Day pointed out that the $30 price means there is only $2 to $4 dollars for the electronics, so two design rules come into play: do it in firmware, not hardware; and make the printed circuit board (PCB) an integral part of the design, not just a platform the components.

Let's first look at the underlying specifics. The remote control link is not RF, it's done via infrared, and has a range of about 25 feet (7 to 8 meters). A trio of IR light emitters in the handheld controller disperses a 2 kHz, PWM, non-return to zero (NRZ) data stream, with one 20 msec packet every 150 msec. A checksum is also added so the chopper can verify, to a first level, that the data received is OK. This data includes throttle position, tilt position, and left/right trim information. All data formatting and protocol is done in firmware, without any dedicated hardware.

The controller also includes a charger for the helicopter's lithium-ion battery pack, and works with the pack to manage charging and provide critical safety oversight. The CPU power of the handset is minuscule, just a Microchip PIC16F505 microcontroller with 1Kcode space, 72 bytes of RAM, and no interrupts; the code runs a continuous service loop. When the CPU senses that the controller is connected to the battery pack, it switches itself from "fly and control" mode" to "charge and manage" mode.

The PCB is the start of the "out of the box" design thinking. When the user squeezes the throttle, he or she gets the sense of a continuous speed control, such as via a potentiometer. But the actually throttle position is developed using a 16-step grey code set of tracks on the PCB, and the throttle just shorts out successive tracks as it is pressed. The microcontroller determines which tracks are shorted out and thus knows the throttle position.

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