2 Cell LiPo Low Voltage Warning Circuit

Transcription

1 In the very early days of radio control, electronics was a big part of the hobby. Many people built or integrated their own radio systems. Now, very reliable and relatively cheap radio systems are available. There are now few opportunities to incorporate the electronics hobby into model aviation (with the notable exception of FPV, although that is only an integration opportunity for most people). I recently identified a do it yourself (DIY) electronics application for model aviation. I am setting up a Discus Launch Glider (DLG) with a 2 cell LiPo to power the radio gear. It is a challenge to make sure that the battery does not go below a safe voltage. Telemetry systems are available, but that involves additional cost and weight. I could set a conservative time limit, but then I would have to track the time the radio is on. Instead, I have put together a circuit that monitors battery voltage and sounds a buzzer when the cell voltage drops below 3.7. How it Works I can t go into too much detail in this format, but here is a conceptual description of how it works. 1. A 3.7 volt voltage regulator circuit provides a reference voltage equal to cell voltage that should sound the alarm. 2. A precision voltage divider provides a voltage equal to half the battery voltage. Since this circuit is for a 2 cell battery, half the battery voltage equals the cell voltage. 3. A comparator circuit compares the reference voltage to the cell voltage (half of the battery voltage). When the cell voltage drops below the reference voltage, the comparator output swings from near the full battery voltage volts to near zero volts. 4. A transistor connected to the comparator output switches on an LED and provides an input to a second transistor. 5. The second transistor switches on the buzzer. The second transistor is required because the comparator does not supply enough current to drive the first transistor with enough current to power the buzzer and the LED. Specifications (Prototype) Weight 5 grams Dimensions board assembly 20mm x 20mm x 12mm buzzer w/ leads 12mm dia. Cylinder x 15mm tall LED w/ leads 5mm dia. cylinder x 10mm tall Current <7.4 volts 20ma 7.5 volts 3ma 8.0 volts 5ma 8.4 volts 13ma Buzzer sound pressure 10cm 1

2 Bill of Materials Ref Description Required Digi Key Part number Cost Q1, Q2 SST2222A transistor 2 SST2222AT116CT ND $0.34 RN1 15K Resistor Network 1 MAX5491LA01000+TCT ND $3.41 U1 TL 331 Comparator ND $0.64 U2 Regulator 1 NCP502SQ37T2GOSCT ND $0.80 C1, C2 1uf tantalum capacitor ND $0.84 R1 4.7K resistor 1/8 watt 1 CF18JT4K70CT ND $0.09 R2 1K resistor 1/8 watt 1 CF18JT1K00CT ND $0.09 LED 20ma LED 1 C566C RFS CT0W0BB2CT ND $0.23 N/A sot 23 3 board CA ND $1.72 N/A sot 23 5 board CA ND $1.92 N/A SC 70 5 board CA ND $1.92 N/A buzzer ND $2.89 $14.89 Note: Datasheets which include pin outs are available at the links above. Circuit Diagram RN1 IN- VCC GND U1 R1 4.7K PIN_3 IN+ OUT PIN_2 Q1 PIN_1 PIN_3 B(-) B(+) + C1 1uF VIN VOUT + C2 1uF R2 1K PIN_2 Q2 PIN_1 Buzzer (+) GND U2 LED1 Buzzer (-) ENABLE 2

3 Assembly This circuit uses very small Surface Mount Device (SMD) integrated circuits. While these parts can be challenging to work with, there are 3 main benefits of doing so: size, weight and most important: availability. Many integrated circuits, especially new ones, are only available in the tiny SMD packages. To make it practical to work with the SMDs, this project employs prototyping boards for the SMD chips. Once you solder the chips to the boards, you can build the rest of the circuit by soldering the other components to the pins on the boards. Always use solder flux, a fine tip soldering iron and very fine rosin core solder. Radio Shack sells.015 solder that works well for this application. The TL331 and the transistors are especially static sensitive. The NCP502 and the precision voltage divider are also somewhat vulnerable to static damage. Wear a grounded anti static wrist strap when working with these components. Note that you can usually get away with less careful handling in Houston s humid climate, but on a dry day or when the air conditioning is working well, you can still zap some semiconductors if you do not ground yourself. Static Damage Awareness Bracelet (Actually an anti static grounding wrist strap must connect grounding wire to snap on buckle) Prototype board subassemblies w/ penny for scale. 3

4 Step by Step 1. Assemble the TL331 prototyping board a. Solder the TL331 (U1) to the SOT 23 6 prototyping board. i. Hold the board in a vise or clip ii. Apply solder flux iii. Position the SMD iv. Hold it in place with a probe v. Solder 2 opposite corners vi. Solder remaining pins vii. Clean solder flux off of board Note: Orient the TL339 with the 3 pin side facing left. 4

5 b. Complete construction of TL331 prototyping board. i. Solder the middle pin of RN1 to the pin 1 trace on the TL331 prototyping board. This connects the center of the voltage divider to the inverting input of the TL331. ii. Use a short section of 30GA wire wrapping wire to connect one of the other 2 pins on RN1 to the pin 5 trace on the TL331 prototyping board. This connects one side of the voltage divider to battery + 5

6 iii. Use the same techniquee to connect the remaining, unconnected pin of RN1 to the pin 2 trace of the TL331 prototyping board. It does not matter which of the 2 end pins on RN1 get connected to pin 2 and pin 5 of the TL331. This connects the other side of the voltage divider to battery. 6

7 iv. Solder a 4.7K resistor between pins 4 and 6 on the TL331 prototyping board (pin 6 of the board connects to pin 5 of the TL331). This pulls the output of the TL331 up. The output of the TL331 is an open collector. It needs the pull up resistor so that it does not float. When the output of the TL331 goes low, it sinks enough current to overcome the current flowing through the pull up resistor so that the attached transistor sees a low. 7

8 v. Solder a short section of 30GA wire wrapping wire to pin3 of the prototyping board. Later you will use this wire to connect tothe non inverting input of the TL339 to the output of the NCP503 voltage regulator board. 8

9 2. Assemble the NCP502 prototyping board. a. Solder the NCP volt regulator (U2) to the SC 70 6 prototyping board using the same general procedure as you used for the TL331. Note: Orient the NCP502 with the 3 pin side facing left. 9

10 b. Complete the NCP502 prototyping board. i. Solder C1 between pins 1 and 2 of the NCP502 prototyping board. The + end (end w/ a bevel on the case and a line across the end) connects to pin 1 (leftmost pin). 10

11 ii. Solder a short piece of 30GA wire wrapping wire to the negative end of C2. iii. Solder the + side of C2 to pin 5 of the NCP502 prototyping board. 11

12 iv. Solder the wire that is connected to the negative end of C2 to pin 2 of the NCP502 prototyping board. 12

13 v. Solder a short section of 30GA wire wrapping wire to connect pin 1 to pin 3 of the NCP502 prototyping board. This connects the enable pin of the NCP502 to battery +.Note: Photo only shows wire connected to pin 1. 13

14 vi. Solder 30GA wire wrapping power/ sense wires to the NCP502 prototyping board. Use wire color or markings to identify polarity. 1. Solder positive wire to pin Solder negative wire to pin 2. 14

15 2 Ceell LiP Po Low w Volltage Warn ning Circuit C t 3. Assemble the transistor t pro ototyping boaard. a. Solderr the two SST2 2222A transisstors to the SOT 23 3 transistor prototyyping board using u the same general procedure p you u used for U1 and U2.Note e: side of tran nsistor with 2 pins points down. 15

16 b. Complete construction of transistor prototyping board. i. Strip the ends of a short length of 30GA wire wrapping wire and use it to connect the pad for pin 1 of Q1 to the pad for Pin 2 of Q2 (pins 2 & 4 of the transistor prototyping board).note: In this photo, the traces for pins 2 and 5 have been cut and removed as directed in the sub steps below. ii. iii. iv. Cut the trace for pin 2 of the SOT 23 3 prototyping board in 2 places near the board pin using a hobby knife. Use your soldering iron to heat the trace that you cut so it will delaminate. Apply a little solder so you get good heat transfer. Once heated, use the hobby knife to remove the piece of the trace. 16

17 v. Solder a 1k resistor across the broken trace. vi. Cut a section out of the pin 5 trace on the transistor prototyping board and use a soldering iron and hobby knife to remove the piece. 4. Attach wires to the buzzer and the LED. a. Use wire color or wire markings to identify polarity. b. The longer lead on each component is the negative side. After identifying the polarity, cut each lead short, leaving 1/8 to 3/16. 17

18 c. Solder 30GA wire wrapping wire pieces to the buzzer and the LED. d. Apply 1/16 or smaller heat shrink tube to insulate the pins. For 1/16 heat shrink, you may have to pinch the tube while it is hot to close the gaps and provide some mechanical support. 18

19 2 Ceell LiP Po Low w Volltage Warn ning Circuit C t 5. Connect the LEED and buzzer to transisto or prototypingg board. a. Solderr both negativve wires to pin 5 of the board (lower po ortion that haas been severred from the t part of thee trace that connects c to trransistor Q2).. b. Solderr the LED posiitive wire to pin p 2 of the board. c. Solderr the buzzer positive p wire to t the cut trace above pin 5 of the boarrd. This is thee trace that t pin 1 of Q2 Q is soldered d to. 19

20 6. Clean the assembled prototyping boards. Subsequent steps involve adhesives, and solder flux will interfere with adhesives. There are also other problems with leaving solder flux on the boards. 7. Plug the prototyping boards into a solderless breadboard and connect per circuit diagram. Test the circuit. Resolve any problems with each subassembly before proceeding. 8. Use clear silicone sealant to secure RN1, C1 and C2. These components need mechanical reinforcement since some of their pins are not soldered to a board or a rigid component. 9. Use epoxy glue to secure wires a. Glue the power wires to the back of the NCP502 prototyping board. b. Glue the LED and buzzer wires to the back of the Transistor prototyping board. 10. Assemble and wire prototyping boards. a. Place a piece of vinyl electric tape over the pins on the back of the TL331 prototyping board. Cover all of the pins except for the narrow part intended to fit into a socket. This prevents shorts when you glue this board to the other prototyping board. b. Glue TL331 and NCP502 prototyping boards together. i. Glue them back to back. ii. Face pins in the same direction. iii. Use silicone sealant. 20

21 c. Connect pins together with 30GA wire wrapping wire. i. Connect pin 1 of the NCP502 prototyping board to pin 6 of the TL331 prototyping board. Pins are adjacent since the boards are back to back. This is the battery + connection. ii. Connect pin 2 of the NCP502 prototyping board to pin 2 of the TL331 prototyping board. Pins are the second pins in on each end since the boards are back to back. This is the battery connection. iii. Connect pin 6 of the NCP502 prototyping board to pin 3 of the TL331 prototyping board. Count pins from the front of each board since the boards are back to back. This connects the 3.7 volt output to the non inverting input of the TL331 providing the reference voltage. d. Glue transistor prototyping board to low voltage alarm prototype board assembly. i. Cover pins on transistor prototyping board as in 10.a. ii. Orient transistor prototyping board pins at 90 to the existing pins on the low voltage alarm prototype board assembly. iii. Glue board to assembly using silicone sealant. e. Connect pins together with 30GA wire wrapping wire. i. Connect pin 4 of the TL331 prototyping board to pin 1 on the transistor prototyping board. Count pins from the front of each board. This connects the TL331 output to the base of Q1. ii. Connect pin 1 of the NCP502 prototyping board to pin 3 on the transistor prototyping board. Count pins from the front of each board. This connects pin 3 of Q1 and Q2 to battery +. iii. Connect pin 2 of the NCP502 prototyping board to pin 5 on the transistor prototyping board. Count pins from the front of each board. This connects the negative leads for the buzzer and the LED to battery. 21

22 11. Board Assembly Diagram Battery + Transistor Board NCP502 Board TL331 Board Bottom View (pins pointing towards you) 2. Transistor board view distorted to show pins 3. LED and buzzer leads not shown 4. Black chip representations indicate front of boards 5. Other components not shown 12. Test the low voltage alarm circuit. 13. Insulate assembly. a. Clip unused pins from the prototyping boards. b. Clip short unneeded portions of remaining prototyping board pins. c. Place strips of electrical tape over pin ends. d. Apply liquid electrical tape to assembly. 22

23 Testing You can test the circuit in any of the following ways: 1. If you have a variable voltage power supply, set it to about 8 volts, connecting to the circuit and decrease the voltage until the circuit emits a tone and the LED comes on. 2. Connect the circuit to a storage charged 2 cell LiPothrough a potentiometer rated for at least 1/4 watt using the following circuit. You are making a variable voltage divider so you can change the voltage the circuit sees. Turn the potentiometer until the circuit emits (or stops emitting) a tone and the LED comes on (or goes off) Cell LiPo 20K ohm Potentiometer Low Assembly

24 3. Connect the circuit to a storage charged 2 cell LiPo and check the voltage periodically as you wait for the circuit to trigger. Add a servo tester and servo and set it to continuous operation (be sure to use a UBEC unless you are using a high voltage servo and servo tester). Running the servo drops the battery voltage faster than just waiting for the 3 13ma draw of the circuit to bring the battery voltage down. Be sure to check the battery voltage periodically to avoid destroying your battery by excessive discharge in case the circuit does not work properly. Hint: Use a very low capacity battery if you have one ( mah). This reduces the time required for the test. Operation Wire the circuit into the 2 cell LiPo battery before any UBEC or voltage regulator. Circuit will begin to chirp at about 7.4 volts. The chirping will transition to a steady tone. By 7.3 volts the buzzer will be on continuously. The LED will also flash and then stay on steady as voltage decreases. 24