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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →These two RC timer circuits produce opposite relay sequences: a CD4050-based on-delay timer switches a load on after a preset delay, while a CD4049 inverter-based off-delay timer starts with the relay energized and switches it off after a delay. Electronics For You reports an adjustable interval of approximately three to fifteen minutes; treat that as an approximate project range, not a guaranteed timing specification.
How the two timers differ
| Feature | CD4050 on-delay | CD4049 off-delay |
|---|---|---|
| Load sequence | Relay and load start off; the load switches on after the capacitor charges to the input threshold. | Relay starts energized; the load switches off after the capacitor charges to the input threshold. |
| Logic action | The CD4050 is a noninverting buffer. Its output goes high when the rising capacitor voltage crosses the input threshold. | The CD4049UB is an inverter. Its output begins high in the described circuit and goes low when the capacitor voltage crosses the threshold. |
| Timing action | C1 charges through R2 and VR1; increasing resistance or capacitance lengthens the delay. | C1 charges toward the inverter threshold; a modified version adds a discharge/reset path, with D1 described as resetting C1 when power is removed. |
| Reported interval | Approximately three to fifteen minutes, adjustable with VR1. | The source describes the same approximate adjustable range; it is not a guaranteed interval. |
| Supply and relay | Source circuit range is 5–12 V; use a relay coil suited to the supply, including a suitable 5 V relay on a 5 V supply. | Source circuit range is 5–12 V; use a relay coil suited to the supply, including a suitable 5 V relay on a 5 V supply. |
| Load and installation | Relay-contact suitability depends on the load and installation. The source does not establish a specific appliance rating or code-compliant installation. | |
TI identifies the CD4050B as a noninverting buffer and the CD4049UB as an inverter: CD4050B product page and CD4049UB product page.
How the CD4050 on-delay circuit works
- Power-up: C1 begins discharged, and the relay is not energized.
- Timing: C1 charges through R2 and the 1 MΩ potentiometer VR1.
- Threshold crossing: Once the voltage at the CD4050 input reaches its switching threshold, the buffer output changes high.
- Relay activation: The output drives NPN transistor T1, specified as a 2N3904, which energizes relay RL1. The relay contacts then switch the connected load on.
Increasing VR1 resistance or C1 capacitance makes the charging interval longer; reducing either shortens it. The reported three-to-fifteen-minute adjustment is approximate. The switching threshold varies with supply voltage, temperature, device characteristics and IC manufacturer, so the circuit should not be treated as a precision timer.
How the CD4049 off-delay circuit works
- Initial state: In the described arrangement, the inverter output is initially high, so T1 conducts and the relay remains energized.
- Timing: C1 charges toward the CD4049 input threshold.
- Switch-off: When the input crosses that threshold, the inverter output goes low, T1 turns off and the relay de-energizes.
The source also describes a modified off-delay arrangement with a discharge/reset path and says D1 resets C1 when power is removed. Reset behavior matters if the circuit is restarted: a capacitor that has not discharged fully may not produce the same delay as one that starts from zero charge.
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Parts and operating details
On-delay version
- CD4050, 2N3904 NPN transistor, two 1N4007 diodes and two 5 mm LEDs
- 4.7 kΩ resistors R1 and R4; 1 kΩ resistors R2 and R3
- 1 MΩ potentiometer VR1
- 470 µF, 25 V and 220 µF, 16 V electrolytic capacitors
- 12 V SPDT relay, connectors, 12 V battery or supply, and push-to-on switch
Off-delay version
The source specifies a 4049 and broadly the same components. Its alternative off-delay arrangement adds a 10 kΩ resistor and changes resistor values. Check the specific circuit diagram and component placement before building; a parts list alone is not a wiring diagram.
The source describes a 5–12 V supply range and recommends matching the relay-coil voltage to the supply, including a suitable 5 V relay for 5 V operation. It describes a 12 V DC switched-mode supply, or a transformer with rectifier and filter, as supply options. Do not assume the relay contacts can safely switch a particular load solely from the relay type: contact ratings and the intended installation must be checked for the actual part and load.
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Why the delay is approximate
The main timing controls are resistance and capacitance, but the threshold at which the logic IC switches is not a fixed timer setting. Electronics For You notes variation with supply voltage, temperature, IC manufacturer and device characteristics. Component tolerances and relay behavior can add further variation. The three-to-fifteen-minute interval is therefore a reported adjustment range for the project, not a verified or guaranteed operating specification.
For an application that requires repeatable timing, test the completed circuit under its intended supply and temperature conditions, measure the actual interval, and decide whether its variation is acceptable. The cited project material does not provide a tested accuracy, relay rating, or suitability determination for any specific appliance.
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Construction and mains safety
The project describes a single-sided PCB for the on-delay circuit, while also allowing Veroboard or general-purpose PCB construction. It suggests fitting the board in an enclosure with panel-mounted connectors, LEDs, switches and potentiometer. These construction options do not certify the circuit or make it suitable for unattended appliance control.
Warning: The relay contacts may carry mains voltage. Never touch the circuit while it is connected to mains. Use proper insulation, an enclosure, earthing, fuse protection and adequate PCB spacing. If you cannot design and verify safe mains isolation and wiring for your location, do not connect an AC load; consult a qualified electrician. The project information does not establish compliance with local electrical codes or suitability for a refrigerator, compressor, pump, geyser or other specific appliance.
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