Low-Power Delay Timer Circuit
Budget: $10 – $30 USD
I’m looking for a discrete-component solution that lets a momentary N.O. dry contact trigger an adjustable delay of 5-60 s (via an off-board potentiometer, only the pads sit on the PCB). After that delay, the circuit must close a second dry contact—driven by a SIL reed relay with its internal flyback diode—for 400-700 ms.
Key objectives
The overriding goal is ultra-low quiescent current so a single CR2032 keeps the unit alive for at least two to three years, even if it fires up to twice per day. No microcontrollers or code are allowed; I want a purely analog or discrete logic approach.
Component guidelines
• Resistors: standard ¼ W values
• Transistors: 2N3904 (or close low-leakage equivalents)
• Capacitors: low-ESR types only
• Relay: SIL reed relay with built-in diode
• Dry contact input and output and potentiometer: These will be remote, so only pads are required on the schematic.
Feel free to recommend exact part numbers within those categories, but please stay within these families. I don’t anticipate needing any contact debouncing on either side of the circuit.
Deliverables
1. Complete, legible schematic (PDF or editable format)
Acceptance criteria
• Idle current low enough to meet the stated battery-life target
• Delay tunable from 5 s to 60 s with the remote pot
• Relay pulse held within 400-700 ms across the CR2032 voltage range
• Reliable operation without firmware or digital programming
If you can craft a rock-solid design that uses only a CR2032 coin cell battery for the power supply, I’d love to review your approach.
Key objectives
The overriding goal is ultra-low quiescent current so a single CR2032 keeps the unit alive for at least two to three years, even if it fires up to twice per day. No microcontrollers or code are allowed; I want a purely analog or discrete logic approach.
Component guidelines
• Resistors: standard ¼ W values
• Transistors: 2N3904 (or close low-leakage equivalents)
• Capacitors: low-ESR types only
• Relay: SIL reed relay with built-in diode
• Dry contact input and output and potentiometer: These will be remote, so only pads are required on the schematic.
Feel free to recommend exact part numbers within those categories, but please stay within these families. I don’t anticipate needing any contact debouncing on either side of the circuit.
Deliverables
1. Complete, legible schematic (PDF or editable format)
Acceptance criteria
• Idle current low enough to meet the stated battery-life target
• Delay tunable from 5 s to 60 s with the remote pot
• Relay pulse held within 400-700 ms across the CR2032 voltage range
• Reliable operation without firmware or digital programming
If you can craft a rock-solid design that uses only a CR2032 coin cell battery for the power supply, I’d love to review your approach.