![]() The two resistors, one at the base of BC547 and the other at the collector of BC557 decide the other threshold of the relay changeover, meaning the full charge cut off threshold of the battery. The modified design for implementing the above condition can be seen in the following diagram: In order to rectify this, a hysteresis effect will need to be introduced which can be accomplished through a feedback resistor between the collector of the BC557 and the middle BC547 transistor. However, this still wouldn't stop the relay from constantly switching ON/OFF at the low battery changeover threshold. This can be prevented by adding a 100uF at the base of the middle BC547. One drawback of the above design could be the chattering of the relay at the threshold voltage levels, due to the battery voltage dropping immediately during the relay changeover process. However when the battery is in its normal state the relay connects the battery with the load and allows the load to operate through battery power. Referring to the above diagram, the low battery indicator is formed by the two NPN transistors, while the additional BC557 and the relay are used for cutting OFF the battery from the load when it reaches the lower threshold, in this state the relay connects the battery to the available charging input. ![]() Upgrading the above Transistorized Low Battery circuit into a Low Battery Cut-off Circuit The LED switches ON and provides the required indications of a low battery warning. This situation instantly switches ON the right hand side transistor, enabling the LED to illuminate. As long as this transistor conducts, the second transistor on the right is held switched OFF, which also keeps the LED switched OFF.Īs soon as the battery voltage falls below the set threshold level, the left transistor is no longer able to conduct. The first transistor on the left senses the threshold voltage level as per the setting of the 47K preset. Circuit can Operate at 1.5VĪnother advantage of this circuit is it's ability to work even at voltages around 1.5V which gives it a clear edge over the IC based circuits.Īs shown in the following circuit diagram, the two transistors are configured as voltage sensor and inverter. Thus the present circuit becomes more efficient especially in cases where stand by current consumption tend to become an issue, example suppose in units which depend on low current battery supplies such as a 9V PP3 battery. The main advantage of the proposed two transistor low battery indicator circuit is its very low current consumption compared to the IC counterparts which consume relatively higher currents.Ī IC 555 would consume around 5mA, a IC741 around 3 mA, while the present circuit would just consume around 1.5mA current. You may also this Low Battery Alarm Circuit However the following post relates yet another similar circuit which is much cheaper and employs just a couple of NPN transistors for producing the required low battery indications. Voltage divider. That is why there are so many on the board.We have so far seen how to make a low battery indicator circuits using a 741 IC and a 555 IC, which are no doubt outstanding with their abilities of detecting and indicating low battery voltage thresholds. I used resistors I had already had, added in series to make up the resistances needed for the ItĬonumes only 4 micro-amps as opposed the the milli-amps my circuit was consuming. The LTC1440 Low Power Comparator IC is available at Digi-Key. ![]() I did it this way so it would all be attached to the back The best thing about it is that it runs on a single 9V supply (the V2's 6 D Cells) and canīe connected directly to the Robosapien V2's power switch leads. This circuit is designed to monitor the voltage in a 9V system. Mr.Johnson was generous with advice and tips which pointed me in the right direction. That required a seperate 9V battery and it had some flaws in the design and implementation. He provided the final circuit design which is available below. ![]() First, I'd like to give thanks and credit to Dave Johnson at for all the help. ![]()
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