Nuts & Volts - February 2006

■ FIGURE 4

+5V

normally used for two purposes:

CAL 2.5K

IR Remote Repeater

8

7

3

6.8K

6 555

The problem is that the battery’s data sheet does not give the recommended charging rate, but does suggest using a “constant voltage-current limited” charger. Could you please shed some light on this charging method, and perhaps provide a circuit to safely charge this battery?

4

2

15

— D.E. Baum Cottonwood Heights, UT

.01

4.7K

Radio Shack ₂276-640 ₁1K IR In ^~_~_1K

IR Rc'vr

RUN

2N2222A

CAL

1) If you leave an input floating, it may oscillate or come to some indeterminate state which can cause 220 high-current thermal runaway in CMOS. It’s _IRgenerally wiser to pull LED the input up rather than ground it. (You’re going to get a lot of opinions on this. Ground level normally has less noise than Vcc, but can dissipate more power.)

3

LED1

what value the resistors are?

— Mack

AVery good question, and one that needs more than a yes/no style answer. Your specific microcontroller has programmable pull-downs. That is, you can ground these pins via software. Other controllers — like the PIC — support software programmable pull-up resistors. The problem is that these are weak resistors — they have no power handling capabilities.

External pull-up resistors are

2) A hard pull-up makes for faster switching time. Why? Because all

devices have a capacitive element — a “built-in” capacitor that has to be charged and discharged when switching from on to off. By putting an external pull-up resistor on an input or output port, this latent capacitor will charge or discharge faster. Oh, before I forget, I generally use a 10K resistor.

HOW DO I CHARGE IT?

QI recently purchased a sealed rechargeable 12-volt 1.2 Ah battery to replace the dry type in an R/C transmitter.

Charge

■ FIGURE 5

Charge

1K

Reed Relay

LM317

T

R1 10 ohms

78L05

0.1

18-volt Wal-Wart

.01

OUT IN

COM

IN OU

COM

0.33 1K

5V

ABecause they are sealed and more fragile than flooded (wet) lead-acid batteries, gel-cells should be lightly charged by the 1/10th method — as in 1/10th of the amp-hour (Ah) rating is used to charge the battery for up to 12 hours, after which the battery is disconnected from the charger. This is especially true for gel-cells under 6 Ah. For your battery, that equals 120 mA of charging current until the battery reaches 13. 6 volts. The following circuit, Figure 5, is semi-automatic in that respect.

The charger is nothing more than an LM317 voltage regulator — configured as a constant-current source — controlled by a latching cut-off switch after the battery is fully charged. To start charging the gel-cell, press the Charge push-button and stand back. This will engage the reed relay, light the Charge LED, and direct charging current into the battery. When the battery voltage reached 13. 6 volts, the comparator output goes high, the relay drops out, the charging stops, and the Charging LED turns off. (Notice, there is no number on this op-amp/ comparator chip. Whatever you have in your junk box or can find at RadioShack will work, i.e., LM339, LM741, etc.)