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A CD4050-based on-delay timer waits before energizing a relay; a CD4049-based off-delay timer starts with the relay energized and turns it off after a preset interval. Both examples use a resistor-capacitor timing network, so their delays are approximate rather than precision intervals. The described project reports an adjustable range of about three to fifteen minutes, but that is not a guaranteed specification.
How the two timer circuits differ
The key difference is what happens to the load while the timing capacitor charges. In the on-delay circuit, the load starts off and turns on after the delay. In the off-delay circuit, the relay is initially on and turns off after the delay.
| Feature | On-delay: CD4050 | Off-delay: CD4049 |
|---|---|---|
| Initial relay state | De-energized while the capacitor charges | Energized initially |
| Action at the input threshold | Output goes high, driving the transistor and energizing the relay | Inverter output goes low, turning off the transistor and de-energizing the relay |
| Logic function | Noninverting buffer | Inverter |
| Timing element | Capacitor charges through R2 and VR1 | Capacitor charges to the inverter input threshold; a modified version adds a discharge/reset path |
| Reported delay | Approximately three to fifteen minutes, as reported for the project; actual timing is not guaranteed | |
TI identifies the CD4050B as a noninverting buffer and the CD4049UB as an inverter. Their product pages link a shared datasheet: CD4050B and CD4049UB.
How the CD4050 on-delay works
- Power is applied: C1 begins discharged, and the relay is initially off.
- C1 charges: Current flows through R2 and the adjustable potentiometer VR1. The RC network determines how quickly the capacitor voltage rises.
- The threshold is reached: When the CD4050 input voltage reaches its switching threshold, the buffer output goes high.
- The relay switches: The output drives NPN transistor T1, energizing relay RL1 and switching its connected load on.
Increasing the resistance in the timing path or increasing capacitance lengthens the wait; reducing either shortens it. The project parts list includes a CD4050, 2N3904 transistor, 1 MΩ potentiometer, 470 µF 25 V and 220 µF 16 V electrolytic capacitors, two 1N4007 diodes, two 5 mm LEDs, 4.7 kΩ resistors for R1/R4, 1 kΩ resistors for R2/R3, a 12 V SPDT relay, connectors and a push-to-on switch. Confirm each component’s placement against the circuit diagram before construction; the parts list alone does not specify every connection.
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How the CD4049 off-delay works
- At startup, the relay is energized: The circuit uses the CD4049 inverter so its output is initially high and T1 keeps the relay on.
- The timing capacitor charges: C1 rises toward the inverter input threshold.
- The inverter changes state: Once that threshold is crossed, the output becomes low.
- The relay turns off: T1 stops conducting, the relay de-energizes and its contacts change state.
A modified off-delay arrangement adds a discharge/reset path; the project describes D1 as resetting C1 when power is removed. That matters because the capacitor’s starting voltage affects the next timing interval. The off-delay parts list uses a 4049 and broadly similar supporting parts, but the source also describes changed resistor values and an added 10 kΩ resistor for its alternative arrangement. Follow the intended version’s schematic rather than assuming the on-delay component values transfer unchanged.
Choosing the supply and relay
The project describes operation from a 5–12 V supply and lists a 12 V battery or supply with a 12 V SPDT relay. If operating the circuit from 5 V, it says to use a relay with a suitable 5 V coil. The supply voltage must match the relay coil rating, and the IC and other parts must be used within their ratings. A 12 V DC switch-mode supply is suggested; a transformer with rectifier and filter is another described option.
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Why the delay is approximate
The project reports a continuously adjustable delay of approximately three to fifteen minutes through VR1. It is an article-reported design range, not a guaranteed timing specification or independently verified measurement. Timing depends mainly on resistance and capacitance, but the logic IC’s switching threshold varies with supply voltage, temperature, device characteristics and manufacturer. Component tolerances and relay behavior also affect the result, so a preset position should not be treated as an exact countdown.
If an application requires a repeatable or safety-critical delay, this RC circuit is not established as suitable for that purpose. The project does not provide verified threshold timing, relay ratings for a particular load, or test results under defined conditions.
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Construction and mains safety
The project suggests a single-sided PCB for the on-delay version or construction on Veroboard/general-purpose PCB. It also describes fitting panel connectors, LEDs, switches and the potentiometer in an enclosure. These are construction suggestions, not evidence of certification or compliance with local electrical codes.
Relay contacts may carry mains voltage. Never touch the circuit while it is connected to mains. If switching an AC load, provide proper insulation, a closed enclosure, earthing, fuse protection and adequate PCB spacing between mains and low-voltage sections. Relay contact ratings and local code requirements are not established here; have the design and installation assessed by a qualified person. The prototype description does not establish suitability for unattended control of a refrigerator, compressor, pump, geyser or other appliance.
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