You can make a simple adjustable delay with either a CD4050B or CD4049UB, a resistor-potentiometer-capacitor network, and a transistor-relay driver. The key difference is output polarity: the CD4050B is noninverting, so the example delays turning a load on; the CD4049UB is inverting, so its example delays turning a load off. Neither is a precision timer—the RC network sets an approximate interval, and the logic input’s switching threshold affects when the output changes.
What is the difference between a CD4050 and a CD4049 timer?
They are not timer ICs; they are six-channel CMOS logic buffers. Texas Instruments describes the CD4049UB as an inverting hex buffer and the CD4050B as a noninverting hex buffer. In the example circuits, that polarity difference determines whether the relay is energized before or after the RC input crosses its threshold. Both parts have a specified operating supply range of 3 V to 18 V. TI’s Rev. L datasheet also lists logic-level conversion among their applications.
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| Example | Logic part | Initial load state | What happens after the RC delay |
|---|---|---|---|
| On-delay | CD4050B, noninverting | Relay and load start off | The relay energizes and the load turns on |
| Off-delay | CD4049UB, inverting | Relay starts energized | The relay de-energizes and the load turns off |
The behavior above describes the arrangements in the cited project, not every circuit using these ICs. Wiring, transistor stage, relay contacts, and any reset path affect the actual sequence.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsHow the RC delay makes the output switch
A capacitor voltage changes gradually as it charges through resistance. The logic input sees that rising voltage and changes state when it reaches the device’s switching threshold. The chip then produces a digital output, which controls a transistor that switches the relay coil. The RC network creates the delay; the buffer’s threshold determines when that delay ends.
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CD4050B on-delay sequence
- At power-up, C1 is initially discharged, and the relay is off.
- C1 charges through R2 and adjustable potentiometer VR1.
- When the CD4050B input reaches its switching threshold, its noninverting output goes high.
- The output drives transistor T1, energizing relay RL1 and turning on the connected load.
CD4049UB off-delay sequence
- Initially, the inverter output is high and the relay is energized.
- As C1 charges through the RC path, the voltage at the CD4049UB input rises.
- Once that input crosses its switching threshold, the inverting output goes low.
- The transistor turns off, the relay de-energizes, and the load turns off. The cited design also describes a modified version with a reset/discharge path.
How to adjust the approximate delay
Increasing the resistance or capacitance slows the capacitor’s voltage change and generally lengthens the interval; decreasing either generally shortens it. The cited project reports an adjustment range of approximately three to fifteen minutes for its particular circuit. That is a project-specific approximate range, not a guaranteed timing specification or tolerance band.
The delay is not simply a guaranteed value obtained by multiplying resistance by capacitance. The transition depends on the input threshold, which varies with supply voltage, temperature, manufacturer, and device characteristics. The cited project notes these variations, and the TI datasheet provides device operating conditions rather than a precision timer accuracy specification. Treat the circuit as an adjustable delay, not a calibrated timer or a safety-critical control.
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Parts in the cited examples
The project’s parts lists are examples, not universal bills of materials. The on-delay list includes a CD4050, 2N3904 NPN transistor, 1N4007 diodes, red and green LEDs, 4.7 kΩ and 1 kΩ resistors, a 1 MΩ potentiometer, 470 µF / 25 V and 220 µF / 16 V electrolytic capacitors, a 12 V SPDT relay, and a 12 V supply. Its off-delay list uses a 4049 instead of the CD4050 with the same general set of parts. See the project article for its circuit diagrams and component placement.
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- C2 is included to help prevent relay chatter.
- S1 provides a discharge path for C1 so the circuit can be reused sooner.
- A diode across the relay coil protects the transistor from coil back-EMF in the modified circuit.
- For the article’s 5 V operation note, it recommends a suitable 5 V relay rather than the listed 12 V relay.
Practical setup and safety checks
- Match the supply and relay coil. The IC’s 3 V to 18 V operating range does not mean a relay coil can use any voltage in that range. Use a relay rated for the supply and a transistor driver appropriate to the coil.
- Follow the input conditions. TI’s datasheet discusses input levels above VCC in the context of logic-level conversion, but its application procedure also warns that CD4049UB inputs in the shown application are not overvoltage tolerant and must remain below VCC because of input clamp diodes. Do not treat the level-conversion feature as blanket permission to overdrive arbitrary inputs.
- Keep logic inputs defined. Tie unused gates to a defined logic level rather than leaving their inputs floating. Respect the datasheet’s recommended input rise/fall and logic-level conditions, keep outputs from being pulled above VCC, and stay within output-current and device-power limits.
- Handle relay contacts as a separate hazard. The relay contacts may switch mains voltage even though the timer circuit uses low-voltage electronics. Use proper insulation, a suitable enclosure, earthing, fuse protection, and adequate PCB spacing. Do not work on exposed mains wiring while powered.
For component sourcing, verify the exact CD4050B or CD4049UB part number, package, and pinout against the datasheet before wiring. Package variants may not share the same physical layout.
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