Activating a vibrator alarm/ Problems with CR2032 battery

[payretep]

Hi all, I'm a new guy from the little island, Singapore. Glad to be in this forum with all the electronics and electrical specialist around.

My problem is as follows:
I have designed a wireless alert circuitry that will generate an alarm signal. I have got my hands on a miniature DC motor as a vibrating alarm but when i wire it to the alarm signal, it doesn't work. Any ideas on why it doesn't work? Also, the alarm signal is an active high signal.

My other problem is with the 3V CR2032 batteries. I have connected 2 batteries is series to obtain a 6V supply. When not connected to any circuit, my multimeter gives a 6V reading, but when I connect the batteries to any load (even a single LED) the voltage drops down to 1.8V. How come the batteries react in such a way and is there any way of making work?

Thanks for taking time to read my problems~

 

[Nigel Goodwin]

Re: Activating a vibrator alarm/ Problems with CR2032 batter

My other problem is with the 3V CR2032 batteries. I have connected 2 batteries is series to obtain a 6V supply. When not connected to any circuit, my multimeter gives a 6V reading, but when I connect the batteries to any load (even a single LED) the voltage drops down to 1.8V. How come the batteries react in such a way and is there any way of making work?

If you're putting an LED directly across the batteries it will pull it down to about 1.8V (or blow the LED with a bigger battery), you need a series resistor to limit the current.

Small batteries tend to have a fairly high internal resistance, they can't provide much current - these sorts of batteries are intended for designs only requiring a few milliamps.

 

[payretep]

Thx Nigel,

So does that mean that I have to use the AAA or AA type batteries? Cos i really hope to cut down on size and weight of the portable unit while having a stable 5V supply (for TTL circuit).
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[Nigel Goodwin]

So does that mean that I have to use the AAA or AA type batteries? Cos i really hope to cut down on size and weight of the portable unit while having a stable 5V supply (for TTL circuit).

I've never played with those batteries, but I doubt they would be very good for feeding TTL chips, they are very current hungry. Using CMOS chips would be better, they use far less current - but you will need to keep current requirements very low.

Try loading them with different value resistors and monitor the current and voltage, you will soon find out their capabilities.

 

[Crofty]

Hi.. If size is critical? Try using 4 x LR44 Alkaline button cells (1.5V @ 175mA) or SR44 (Silver-Oxide) 1.55V. 4 of these will have way more power than CR2032's as these are only designed for low current backup type requirements, mem, clock chips etc. Plus the LR44's are way cheaper TTL? isn't there a CMOS equivalent?? what IC's are you using? there maybe better alternatives?

hope this helps

Steve

 

[payretep]

Thanks to Nigel and Crofty!

I will look into the LR44 batteries and at the same time, I would like to share how my friend solved the DC motor problem. As DC motors require lots of current to drive, the signal derived from the logic chips were not enough to drive it. So my friend suggested a transistor thingy (TIP121) that worked really well. Hope that helps anyone facing the same problem as me.

 

[Crofty]

As an addendum to the above, There is another sneaky trick, If the DC motor is only used intermittently.... A super cap. If the supply can be arranged to charge a 5.5V 1 Farad cap, when the motor kicks in, the cap would supply most of the current, this allows the use of smaller bateries. This is now common practice.

Note a TIP121 is ...er.. physically a bit Big? what current does the DC motor pull?? I would suggest a tiny N-Channel MosFet, which have much lower gate drive requirements than the above power transistor, further reducing power.

Steve

 

[payretep]

Hi Crofty,

I'm interested in the method of using the capacitor for the motor setup, but I really have no idea on how to connect it. So can I ask a favor from you to sketch a simple schematic on the setup? Thanks alot!

The DC motor is the miniature type that you will find in pagers and cellphones. I have limited equipment so I can't measure the current that the motor pull.

Anyway, does anybody have an idea how does a Fluke 199C measure a current with a clamp probe? I used the normal probe and my readings were like 40+ amps!?! So I guessed i measured the wrong way. I connected the normal probes in serial.

 

[Crofty]

As for the circuit, I don't think there is much of circuit for this, it would just connect across your power rails, much like conventional decoupling, with the following reservations... The initial charging of the cap pulls a lot of current from the battery. Plus, the switch for the circuit needs to be after, the battery/cap combination, you don't want to have to charge the big cap every time you switch on, if its discharged through your circuit.

One way to overcome this problem, fit everything 'Except' the battery, then 'pre-charge' the cap from an external PSU/bigger battery to the correct voltage, 5.5V? when the cap volts, fit the batteries. Note if your using 6V batts? you will need a diode dropper arrangement, to ensure that the 5,5V 1F cap doesn't get more than 5.5V (I think most can handle 5.7V peak)

2nd way, use a current limiting resistor to charge the cap, this avoids the heavy inrush current. The circuit then runs off the cap, via a switch.

Note I've haven't tried this yet, as my application is direct solar powered, and doesn't have this problem.

Maybe some other expert here may have a more precise solution? These caps are a bit new technology, and are usually feed with a bit more power than LR44 batts. But I think the theory is reasonably sound.

Steve

 

[Nigel Goodwin]

These caps are a bit new technology, and are usually feed with a bit more power than LR44 batts.

Not very new, they used to be used in old cheap VCR's, 1980's sort of vintage - a common fault was the capacitor failing, which as well as stopping any tuning memory also prevented the micro-processor reseting, killing the machine.

 

[Crofty]

Yup Nigel, I know they've been around, in there basic format for ages, I should have clarified a bit. The ones I'm refering to are the New 'UltraCaps' ie Maxwell's new ones, these are tiny, 5F & 10F, about size of a fat postage stamp, the early ones were not much good at hi-current backup, mainly lo current, mem/clock stuff. The new ranges are much better at supply hi-current bursts, being specifically used to decrease battery size etc, The really big ones now being used in vehicles/electric trains etc for regenerative breaking and boost starts and stuff.

Steve