3.3 FAIL-SAFE CIRCUIT 
The fail-safe circuit is included to insure against loss of control of the model power 
due to failure of control circuit equipment or signal dropout due to multipath reflections. 
A switch on the propulsion-control panel marked POWER AND FAIL-SAFE may be used 
at any time to cut off all model power. This switch controls the power to relays which control 
the outputs of the tone oscillators used for controlling the direction of rotation of the propulsion 
motors. The tones are transmitted to the model via the common radio link. 
Two sections of the model equipment contribute to the fail-safe circuitry. The automatic 
gain-control voltage of the receiver is used to hold a relay latch. Loss of RF signal due to 
either transmitter or receiver failure, or signal loss due to signal cancellation by multipath 
reflections causes a relay contact to short out the input to the tone detector circuitry. The 
tone detector circuitry is arranged so that any one of the four tones must be on to hold the 
fail-safe relay closed. This relay is then used in latching circuits for the motor-alternator 
and motor-generator units which are described in the following section of this report. All 
latching circuits are arranged so that once the command signal has been removed or lost, it is 
impossible to restart the equipment without the use of the alternator and generator starting unit. 
3.4 SPARE TONE CHANNEL 
A spare tone channel has been included in the control circuitry to provide remote 
operation of relay-controlled circuits. This can be used for operation of calibration circuitry 
or other special equipment. The capabilities of this channel can be easily expanded by in- 
corporating a stepping type of switch in the model. 
A switch and pilot light are mounted on the power control panel of the console. When 
the switch is turned on, a tone is transmitted over the radio link which operates a relay in the 
model equipment. The opening or closure of these relay contacts may be used to control any 
desired function. 
4. POWER AND POWER-CONTROL EQUIPMENT 
4.1 GENERAL 
The selection of the equipment was made after considerable searching, comparing, and 
conferring. Some components are production items, others are modified stock items, and a few 
are special designs. The primary consideration in the acquisition of the model-borne equip- 
ment was minimum weight. The motor-generator sets, the alternator sets, and the 10-ampere- 
hour batteries were specially designed for the minimum-weight application. 
