591
VOLUME 33, NO. 11, NOVEMBER. 18%
AN AUTOMATIC STILL-HEAD TIMER 1.0. BINDER Montana State College, Eozeman, Montana
THERE are many places in the laboratory where a device capable of switching electricity on and off in a definite, automatically repeating cycleisuseful. Such a device may be used to control the temperature of a heating oven provided the oven has some heating lag. It can also be used to control photographic printing equipment where a number of copies of the same negative are to he made. Still another use is to activate a solenoid periodically to control the reflex ratio in a distilling column hy causing condensate to be directed to a receiver or returned to the column. When our commercially built automatic still-head timer failed, we devised a makeshift timer that performed very reliably and was considerably more flexible. Although a second model has since been built, most of the parts in the original timer, except the relays, were from war surnlun. With the values indicated in the parts list for C3, C,, R2,R3,Re,and R1the off and on intervals may be preset separately through Reand R1from less than one to about 15 seconds. The values of Ca, C4, Re, and RI may be modified to give different intervals. The external load circuit is closed by the second pair of contacts (not shown in the diagram) on RL,. Pilot lights PI
and P2are not necessary to the functioning of the timer, but are useful in adjusting the cycle visually. By adding an SPST switch to short out the contacts on RL* and a locking push-button switch in the line between the plate of Va and the SPST switch, the timer may be operated as a simple interval timer when desired. For automatic operation the SPST switch should be open and the push-button switch locked. Even if all parts including tubes and chassis are purchased new from a radio supply house, the cost will be under $20. Parts List
C,, C2 = 20 mfd. electrolytic, RL,
= DPDT plate circuit relay, 5WO-ohm coil 450 w. v. C* C. = 1 mfd. paper tubular, RL2 = SPDT plate circuit relay, 5000-ohm coil 400 w. v. R, = 2 2 k h m line cord resistor' R1, RI, R,, Rs = 100,000 ohm, P,,PB = 0.25-watt neon pilot watt light Ra R, = 5-megohm potentiom- V I ,V2,VI = 6V6 GT/Gtube S W = SPST a.-c. line switch eter ~ o m x . chassis, ) tube sockets. Miscellaneous: 9- X 5- X 3-in. (.a.. hsrdware. ete. 1 This may be replaced hy a. 6.3-volt filament transformer, in which case the filaments should be wired in parallel instead of in series, as shown in tho diagram.
