Relay Circuits

This is an accurate long-duration time delay driver, switchable between 1 and 10 minutes or 10 to 100 minutes and whose function does not depend on electrolytic capacitors. Film dielectric caps have been selected. IC1 is configured as a free-running astable multivibrator which frequency is divided down by IC2, a 14020 (or 4020) CMOS 14-stage, ripple-carry binary divider.

IC2 (4017) is a CMOS decade counter IC, a five-stage Johnson counter with 10 decoded outputs. Inputs include a CLOCK, a RESET, and a CLOCK INHIBIT signal. This IC, together with IC3 and the 3-step rotary switch, can provide a maximum division ratio of 81,920 making it possible to time for periods up to 20 hours or so.

C4 may not be needed. Add only if relay 'chatters'. Value can be up to 0.001uF. Led will be lit when relay is "on".

The circuit as shown act as a light sensor. Under normal conditions the resistance of the LDR is high, keeping pin 2 low. When light falls onto the LDR the resistance drops to a couple hundred ohms and triggers pin 2 high which biases the base of Q1 via pin 6 and R4 and in turn activates the relay.

Use this circuit instead of a standard on-off switch. Switching is very gentle. Connect unused input pins to an appropriate logic level ('+' or '-'). Unused output pins *MUST* be left open! First 'push' activates the relay, another 'push' de-activates the relay. IC1, the MC14069 (or 4069) is a regular Hex-inverter type and is constructed with MOS P-channel and N-channel enhancement mode devices in a single monolithic structure. It will operate on voltages from 3 to 18 volts, but most applications are in the 5 to 15 volts.

This circuit does not require clamp diodes. Unlike in the traditional approach, the relay coil is never open-circuited. The CMOS output has either low impedance to ground or to 5V, except for the few nanoseconds during transitions (during which time, stray capacitance easily handles the 20-mA coil current). The 74ACT174 hex flip-flop has ample capability to drive the Aromat TQ2E series of latching relays. Both windings for the TQ2E-L2-5V two-coil relay are on the same magnetic element, so the windings' fields add.

Contact bounce in those relays, however, can prove troublesome to downstream circuitry. One approach to contact bounce combines a relay with a hot-swap controller. Such controllers are increasingly popular as the means for switching system components without shutting down the system power.

An opto-triac is the easiest way. Digi-Key sells them for 1-2 bucks apiece. The sensitivity is in the 5-25mA range, but you could probably coax the 8255 to *sink* that much (ie, drive the internal LED between +5v and the IC pin; to turn on, write-out a zero). Put a 220 ohm resistor in series with the LED.

None of the parts are critical and easy available. The potmeter adjust the trigger 'on' level. The diode in the diagram shows to be 1N914. This is ok if you have a light-duty relay, also the 1N914 is a signal diode so actually does not qualify. Use a 1N4001 (or better) instead. A couple of substitutes for the 2N2222 transistor are: NTE123A, ECG123A, PN100, etc.

The above circuit is similar to the "Delayed Turn-on" type but now configured as an automatic turn-off type. The diagram shows how the circuit function can be reversed so that the relay turns on when power is applied but turns off again automatically after a preset delay. This response is obtained by modifying the relay-driving stage for an NPN transistor like the 2N3904.