Relay Circuits

The first circuit energizes the relay when the temperature rises above the preset level. The second circuit energizes the relay when the temperature falls below the preset level. The two circuits are practically identical. The only difference between them is the polarity of the transistor. The value of the thermistor is not critical. The important thing is the voltage on pins 5 & 6. Any value thermistor should work satisfactorily. But you may need to change the value of R1 - to achieve the desired range of adjustment.

The first circuit energizes the relay when the light rises above the preset level. The second circuit energizes the relay when the light falls below the preset level. The two circuits are practically identical. The only difference between them is the polarity of the transistor. The value of the LDR is not critical. The important thing is the voltage on pins 5 & 6. Any value LDR should work satisfactorily. But you may need to change the value of R1 - to achieve the desired range of adjustment.

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.

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.

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.

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.

Use nine pieces of hookup wire (or a piece of ribbon cable) to connect corresponding points on the I/O Demonstration Board to the Relay Sub Board. Connect 12 volts DC (smoothed) to the power input points on the I/O Demonstration Board. Solder in as many relays as you intend to use, each with its associated LED, resistor and 1N914 signal diode.

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.

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.

This relay driver boosts the input impedance with a regular 2N3904 transistor (or equivalent). Very common driver. It can drive a variety of relays, including a reed-relay, and is non-latching.

This relay remains dormant until the op-amp activates upon sound via the electret-microphone. I only tested the 2-wire type. The input stage is a regular off-the-shelf 741 operational amplifier and connected as a non-inverting follower audio amplifier. Gain is approximately 100 which you can raise by increasing the value of R2. The amplified signal is rectified and filtered via C3, D1/D2, and R3 to an acceptable DC level. D1 and D2 can be any signal diode like 1N914, 1N4148, or the NTE519.

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 shows a pair of CMOS NOR gates which form a push-button-activated one-shot multivibrator relay-switching circuit that provides delays up to several minutes with resonable accuracy. The relay turns on as soon as the START switch S1 is closed.

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.

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.

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.

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.

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.