help simplifying a circuit to trigger beeper when what battery disconnects... - Page 1
Forum section: electronics / projects

author: bigjoncoop
author_rank: Regular Contributor
timestamp: August 26, 2021, 02:27:32 pm
content: starting up a old project of mine for a RC lost model alarmthe circuit triggers a beeper to go off when the main quadcopter batter get ejected,the way these work is , they have a small self contained battery and it monitors 5v coming from the quadcopters flight controller. when that 5v isn't present it uses its own battery to set off a beeper...seems to be overkill to use a attiny85 to do this... couldn't it be done using just a transistor or 2 or a comparator etc...

author: johnkenyon
author_rank: Regular Contributor
timestamp: August 26, 2021, 02:41:36 pm
content: Sounds a bit like the functionality you get with solar powered lights - as long as there's light (=voltage from solar panel), the LED is turned off, lose power and the LED turns on.Replace "solar cell" with diode protected feed from the LiPo (to prevent back feeding), and LED with a buzzer circuit, put an on-off switch to stop the thing from beeping/battery draining when the device is in the cupboard, and that just leaves you to concentrate on a low power driver for the beeper.Maybe repurpose a PCB from a pound shop/dollar store solar light?

author: Caliaxy
author_rank: Frequent Contributor
timestamp: August 26, 2021, 04:05:30 pm
content: Challenge acceptedHow about this?

author: fourfathom
author_rank: Super Contributor
timestamp: August 26, 2021, 05:27:16 pm
content: Quote from: Caliaxy on August 26, 2021, 04:05:30 pm

author: Caliaxy
author_rank: Frequent Contributor
timestamp: August 28, 2021, 05:52:14 pm
content: Nothing critical - whatever you find in your drawers. Any generic diode (e.g., 1N4001) and PNP transistor (e.g., 2N3906) would do it. Also, could follow fourfathom's good suggestions. Note that as drawn, this circuit will try to provide backup power to your drone from the buzzer battery while the main battery is disconnected or gets drained (in addition to sounding the buzzer).Make sure you use an active buzzer (that only requires power, i.e. not PWM). "Load" is the circuitry of your drone. The 10K resistor might not be, indeed, necessary - unless there is some sort of switch in your drone that disconnects the "load" when not in use, preventing it from drawing current from the buzzer battery (left) when the main battery (right) gets disconnected. That would defeat the alarm.Also, note that that the alarm will go off as soon as the main battery is a junction drop (0.6V) below the buzzer battery, not only when the main battery is totally disconnected. You can increase the voltage difference by using a lower voltage battery for the buzzer or by adding diodes in series with the buzzer battery (in increments of 0.6V/diode).

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2021, 07:55:32 pm
content: A small N-MOSFET can also do the job, with virtually no risk of leakage to or from the backup battery:If you like living dangerously R1, D1, and C1 can be omitted, (shorting R1) but then the 2N7000/2N7002 has no ESD protection. LTspice sim attached. N.B. the load must pull the gate up to the rail when the main power goes. If it draws negligible current below some threshold voltage >1V, put a 10K resistor in parallel with it.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 28, 2021, 07:56:33 pm
content: Just don't see reason bothering with "overkill" when a small MCU costs the same as a transistor with diode and resistor. Just put it there and can have features in the future if you find you need them.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2021, 08:03:49 pm
content: Yes, there is that. e.g. a MCU can sleep drawing only uA till activated and then can pulse the beeper to reduce current consumption, directly drive a piezo resonator (maybe + a series choke for more amplitude) efficiently with two pins in antiphase, saving much of the power and weight a cheap active beeper would use, strobe a high intensity LED, and use the same LED as a photocell to determine whether its night or day so it can shut off the LED to save power in daytime, all with minimal external parts. Also it can monitor the data stream from the RC receiver so it doesn't rely on the main battery getting disconnected successfully.