Relay Circuits

This handy little circuit can tell the difference between darkness and light, making it very useful for switching on and off signs, porch lights or other things when it gets dark or light...

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...

Relays are usually specified to pull in within 3 msec at 80% of the rated voltage and to release at 30% of the rated voltage. The circuit in Figure 1 drives as many as eight 12V (½120V coil) power relays, which memory-map into an 8-bit mP bus...

In contrast, the circuit in Figure 1 physically inserts a resistor in series with the load to limit the inrush current and then short-circuits the resistor after a time delay. You can adapt the circuit to any size load by suitably selecting the series resistor and the relay-contact rating...

The circuit`s operating current is low; using the resistor values in Fig 1, the circuit consumes less than 6 µA from a 9V battery. The circuit also features a wide input-voltage range, which depends on the maximum collector-emitter voltage rating of the transistors Q1 through Q3 and the FET Q4...

The circuit is very simple: it uses two transistors with overall feedback to give a high input impedance (470K) and a fast `snap action` drive to the relay. The circuit also includes a few extra bits which are not used in the standard version but enable changes in its operating parameters to be easily made. Full circuit description and values are given, with the circuit board copper layout and more detail in the `members only` area...

The design uses a proximity detector rather than a pushbutton switch to eliminate the need to mount and wire any equipment outdoors. The circuit worked well in this application and other applications...

A time delay relay is a relay that stays on for a certain amount of time once activated. This time delay relay is made up of a simple adjustable timer circuit which controls the actual relay. The time is adjustable from 0 to about 20 seconds with the parts specified. The current capacity of the circuit is only limited by what kind of relay you decide to use...

In his circuit Marin has used two transistors wired as a high gain compound pair. Transistor T1 may be a 2N2222A and T2 a BC108. The current gain will be the product of each transistors beta, which will be a minimum of 140 x 110 or 15400...

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...

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...

This project shows you how to build a relay controller using the Basic Stamp I interfaced to the PC serial port. The Visual Basic 5 software developed for the interface lets you interact with the Basic Stamp to turn ON/OFF up to (2) relays attached to the Basic Stamp I/O pins. As shown below in the screen capture of PC-Relay, it`s easy to select the desired com port using the drop-down menu...

The circuit below uses a CMOS dual D flip flop (CD4013) to toggle a relay or other load with a momentary push button. Several push buttons can be wired in parallel to control the relay from multiple locations...

Many Video Operated Relay (VOR) circuits have been published in recent years for use in connection with ATV repeater controllers or automatic videotape logging systems. Unfortunately, many of these circuits fail to properly detect certain video signals depending on their signal-to-noise ratio or their picture content. Most designs rely on using an LM567 phase locked loop (PLL) tone decoder chip to detect the presence of horizontal sync energy in a composite video signal, and this alone is probably the greatest cause of the poor performance provided by many of these circuits...