Battery voltage monitor
Philips’ IC Type TEA 104 1T is ideal for monitoring the voltage of 1.8_4.0 V batteries. Internal trigger and timing logic prevent a circuit from reacting to brief breaks in the supply voltage caused by pulses on the load current. 6ne or two LEDs may be used to indicate when the voltage drops below a preset level.
A circuit with two LEDs is shown in Fig. 1. Potential divider R1-R2 determines the voltage level below which the LEDs indicate that the battery is charged: The divider should give a voltage of exactly 1.25 V at pin 1. Give R2 a value between 1 Ic.C2 and 100 ii-S2 and calculate R1 from
R1= R2 (Uth/ 1.25-1) [Ω]
where Uth is the wanted voltage level. The values of R3 and R4 are 100-220 CI, depending on the battery voltage. The TEA1041T is enabled when pin 3 is at mound level. If the voltage at pin 1 drops _low 1.25 V, a digital counter runs for about 2 s. if the level at pin 1 remains <1.25 V. the IC goes into the alarm state: D1 lights and continues to do so even when the voltage at pin 1 reaches 1.25 V again. If in this condition S1 is opened, both LEDs flash for about 4 s. After that, the IC reverts to the standby state, in which it draws a current of about 10 pA, which constitutes only a minute a load on the battery.
Figure 2 shows how the IC can be incorporated in an existing apparatus: D2 is omit-ted and pin 3 is permanently at ground level. The monitor is connected to the load as long as S1 is closed. The IC per-
forms as described earlier, but the LED does not flash after the reset.
If the supply voltage is higher than 4 V, connections must be made as shown by the dashed lines in Fig. 2 (R1 to R5 and D3 and C3 to earth). The supply voltage is then lowered to 3.3 V by D3. The potential divider is connected to the full supply voltage.