make such circuits with cycle times of several 
minutes. These can be extended with care to 
l hour. Accuracy of $14 minutes per day can be 
obtained. Such a unit might draw anywhere from 
2 to 5 watts maximum power. The price would run 
between $100 and $200. 
Reed Controlled Count Down. A more in- 
expensive version of the tuning fork method is to 
use a vibrating reed which directly makes an 
electrical contact. This circuit closure then 
would be counted down the same as the tuning fork 
frequency. Estimated accuracy on such a unit is 
minutes per day. Life would be dependent upon 
contacts and might be as much as 2,000 hours. 
The estimated price would run $100 to $200. 
MECHANICAL TIMERS 
Possible trouble points in mechanical timers 
are: position sensitivity, temperature compensa~ 
tion, variation of rate with battery voltage, 
life of brushes, life of the rate controlling 
mechanism, power consumption, resistance to shock, 
low torque, generation of electrical interference 
and lubrication, bearing quality and lubrication. 
D.C. motors are used widely as timers. Un- 
governed D.C. motors as a rule cannot be counted 
on for accuracies of better than £2 hours per day 
because of battery life. Also, if the temperature 
in the winding increases, the resistance drop in- 
creases and, therefore, speed will drop. Prices 
on D.C. motors can range anywhere from thirty 
cents to many hundreds of dollars. 
Governed D.C, Motors, (a) Centrifical Speed 
Control. Centrifically operated contacts mounted 
on the motor shaft will break motor power con- 
tacts or switch in dropping resistors as the de- 
sired speed is exceeded. Approximate accuracy 
is t14 minutes per day. This motor would draw 
203 to .06 watts and give 4 ounce inches torque 
at 1 R.P.M. The life will be around 1,000 hours. 
The price can be as low as $17. To construct a 
timing device from a D.C, motor, one must add 
cost for cam, switch, bracket and housing; 
(b) Reed Contact Control. The motor receives 
part of its power through contacts mounted on a 
reed which vibrate at a given frequency. The 
reed passes current to the motor shaft in pulses. 
If pickup on the shaft is not in phase with the 
reed pulse, pulse duration to the motor is 
shortened or lengthened, depending on whether the 
motor is lagging or leading the control speed; 
thus, the length of time that power is applied to 
the motor per pulse is varied as a speed control. 
This method can hold accuracies of 47 minutes per 
day. If it is adjusted closely and voltage 
fluctuations are not too great, accuracies ap- 
proaching +3 minutes per day may be obtained. 
The unit is position sensitive, however. Life 
figures on this unit are incomplete. Life may 
78 
approximate 3,000 hours, but there is some ques- 
tion of contact failure. Power input is 1/2 to 
2 watts. The price of the motor itself is $45. 
The switching assembly would be extra; (c) Chrono- 
metric Contact Control, This unit functions ap- 
proximately the same as the reed contact control 
except that the frequency standard is chronometric 
rather than reed. It is capable of holding 
t8 minutes per day in a temperature range -10° 
to 100°9F. Speed can be held to +2 minutes per day 
in narrow temperature rances. The current re- 
quired by this unit is 250 micro amperes at 
1.5 volts. Points to check in this unit are: 
ability to withstand shock, contact life and 
position sensitivity. Life is estimated to be in 
the order of years, The approximate price is $100 
for the motor without the cam-switch assembly. 
There is another unit available which has higher 
current drains but has better resistance to shock 
and more torque output. Price of this unit is 
$75. Life is guaranteed for 1,200 hours. 
Clock Movements. These are used the same as 
the D.C. motor, i.e., to rotate a cam against a 
micro switch at uniform rate. (a) Impulse Type. 
The balance wheel is given an impulse every cycle. 
This type of movement generally has few or no 
jewelled bearings. The estimated price is $10 to 
$12, not including timing assembly; (b) Solenoid 
or Motor-Winding Type. In this unit a spring is 
wound by a solenoid or motor once every few 
minutes. The average power input of this type is 
approximately 1 milliwatt. Torque output of such 
units covers a wide range--a representative figure 
might be 4 grams centimeters torque at 1 revolu- 
tion per hour. As the movements operate from 
spring power, voltage fluctuations do not affect 
them. The units should be checked for jewelled 
bearings, shock mounting of pivot shaft and 
proper temperature compensation. Accuracies can 
be expected of £10 seconds per day with a life of 
1 to 5 years. This type of movement will cost 
from $12 to $30 with no provision for timing 
assembly; (c) Spring-Wound Movement. In these 
units a heavy-duty clock movement is driven by a 
hand-wound spring. The torque output is sufficiet 
to be used as a circular chart drive. Thirty-one 
day movements can be obtained. Accuracy is es- 
timated at +10 minutes per day. Price of such a 
unit without the timing assembly--$12.50. 
Thermal Timers. A bi-metallic strip is de- 
flected by an electric heater, making or breaking 
a contact. Reset time is determined by how 
quickly the unit cools off. The cycle depends 
upon voltace and ambient temperature. Accuracy of 
about +2 hours per day. Price--$.50. 
Restricted Flow of Mass. A series of timers 
depend upon an orifice restricting mass flow at a 
given rate. An example would be air compressed by 
a solenoid-driven piston and released slowly 
through an orifice. These units provide reliable 
Operation with an accuracy of #2 to 3 hours per 
day but are limited to maximum intervals of 
