Bargraph timer, an unusual LM3914 application

LM3914 and LM3915 are dot/bar display drivers used to create basic displays of analog voltage levels. They are widely used in VU-meters and various voltage indicators (for batteries). The LM3914 senses an analog voltage and drives a number of LEDs depending on the level of this voltage. Knowing this, we can provide this voltage from a discharging capacitor. Depending on the resistive load which discharges the capacitor, the time required to turn off all LEDs can be set to specific intervals. This is how a timer is made using LM3914 (or LM3915).

This IC contains a constant current source for LEDs and an adjustable voltage reference. The following circuit uses LM3914 internal current source to charge the capacitor. To be able to modify countdown time, a potentiometer is used to discharge the capacitor. When the last LED is off two opamps drive a relay. The two opamps are part of the LM358 integrated circuit and one of them is used, in a similar manner, with a capacitor that is charged/discharged to provide an optional turn-off time for the relay. By setting a jumper you can choose between relay always on after time is up or relay on for an adjustable amount of time.

LM3914 countdown timer built on PCB

Volumetric sensor (broken window or opened door alarm)

The following device can detect sudden small air pressure changes in a room caused by opening a door or breaking a window. It is therefore useful to automate lights or as alarm. Although it uses as detector a common electret microphone, the circuit does not react to sound. Used as an alarm, it will not trigger while you are in the room, unlike infrared and ultrasound movement detectors which need to be deactivated when you are home.

I found this circuit in an old electronics magazine and I decided to build it without expecting too much from a microphone based air pressure sensor. The circuit is purely analog, without microcontrollers. Yet, as I found out it does its job very good. Microphone signal is filtered and amplified using common opamps and a couple of 555 timers keep the relay activated for an adjustable amount of time.

Built volumetric sensor