Abstract:
A battery condition indicator and on-off relay circuit for electric guitars with on-board active electronics, adaptable to many other battery-operated appliances, includes an electronic relay circuit capable of disconnecting one or more batteries, under control of a pair of on-off contacts. A test interval timing circuit delays turnoff by about 10 seconds, during which a test of the battery (or batteries) is performed automatically under normal load by comparison to a precision reference voltage. Good battery condition is indicated by illumination of a light-emitting diode which may be located inside the guitar body, visible thru the opening of a standard audio output jack, which may include the on-off contacts, actuated by insertion or removal of the mating audio output plug. The circuit is particularly power-efficient to minimize battery drain.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of portable appliances operated from batteries, and more particularly to the field of electronic musical instruments such as electric guitars. While the great majority of guitars have been fitted with only passive electronic circuits, there has been a trend to incorporate active on-board battery-powered preamplifiers and audio processors to overcome the shortcomings and performance compromises inherent in passive guitar electronics. This trend, as part of a general proliferation of battery-operated appliances, has created requirements, heretofore unsatisfied, for positive, accurate and convenient determination and indication of battery condition. 
     One solution sometimes found on scientific instruments and the like has been the use of a built-in analog or digital voltmeter, operable by a selector switch or pushbutton. However such a solution is unnecessarily costly, complex, inconvenient and confusing to non-technical users such as musicians. Other known battery-testing schemes often conflict with aesthetics, particularly in retrofitting an existing product design, where styling constraints may be imposed precluding drilling the enclosure or adding exterior fittings of any kind, ruling out the use of known systems requiring switches, pushbuttons or exterior-mounted indicators. Known battery-testing schemes requiring manual operation of pushbuttons or switches tend to be disregarded by the user because of the inconvenience of having to perform the test; consequently, unexpected battery failures are not uncommon. 
     Needs have developed for indicating the condition of a pair of batteries used in a dual power supply configuration, and for disconnecting both of them from their loads, controlled by a single pair of contacts, which may be part of an audio jack mounted on an electric guitar, where the contacts are actuated by removal of the plug of an audio cable from the jack. 
     OBJECTS OF THE INVENTION 
     Accordingly it is a primary object of this invention to provide an electronic circuit for indicating the condition of a battery to warn the user that the battery is nearing the end of its useful charge or life expectancy. 
     It is a further object of this invention to provide a battery condition indicator for a pair of batteries in a dual power supply circuit. 
     It is a further object of this invention to provide, in an electronics package for installation into an electric guitar body, a dual-battery condition indication system in which removal of an audio plug from a jack in the guitar body initiates a brief test period, during which acceptable voltage level under normal load is indicated by illumination of a light emitting diode, followed by automatic shutoff of both batteries. 
     It is still a further object of this invention to provide, for electric guitars with active electronics, an automatic battery condition indicator system which can be readily installed into the guitar body without drilling or otherwise altering the style and appearance of the guitar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a battery condition indicator and on-off relay circuit in a preferred embodiment of this invention as applied to a dual battery power supply for an on-board guitar preamplifier. 
     FIG. 2 is a cross-sectional view of a portion of a guitar body showing a battery condition indication light-emitting diode located so as to be visible through the audio jack when the plug is removed, according to an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a first 9 volt battery 1, in parallel with capacitor 2, has its negative terminal connected to common ground and its positive terminal connected to the emitters of PNP power switching transistors 3 and 4. The collector of transistor 3 is connected to the positive supply buss 5 which is connected to the positive power input terminal of on-board guitar preamplifier 6. 
     A second 9 volt battery 7, in parallel with capacitor 8, has its positive terminal connected to common ground and its negative terminal connected to the source of field-effect transistor (FET) 9. The drain of FET 9 is connected to the negative supply buss 10 which is connected to the negative power input terminal of the preamplifier 6. The gate of FET 9 is connected thru resistor 11 to the collector of transistor 4, and thru resistor 12 to the negative terminal of battery 7. 
     The bases of transistors 3 and 4 are connected thru resistors 13 and 14 respectively to the output of NOR gate 15, which has one input connected to timing interval buss 16, and its other input connected to on-off buss 17, which is connected thru resistor 18 to the positive terminal of battery 1 and thru resistor 19 to switch contact 20, engaging contact 21 which is connected thru resistor 22 to ground. Contact 21 also forms the tip contact of audio output jack 23, and is connected thru coupling capacitor 24 to the output of preamplifier 6. 
     On-off buss 17, connected thru capacitor 45 to ground, is also connected to the input of inverter 25, whose output is connected thru resistor 26 and diode 27, in parallel with each other, thru capacitor 28 to ground. The junction of resistor 26, diode 27 and capacitor 28 is also connected to an input of NOR gate 29 which has its other input connected to the on-off buss 17 and its output connected to the test interval buss 16 and to the inverting input of comparator 30. The non-inverting input of comparator 30 is connected to the tap of a voltage divider formed by resistor 31 connected to ground and resistor 32 connected to the positive supply buss 5. The output of comparator 30 is connected thru resistor 33 and 2.5 volt reference device 34 in series to ground. The junction of resistor 33 and reference device 34 is connected to the non-inverting input of comparator 35, and thru resistor 36 in series with resistor 37 to the positive supply buss 5. The junction of resistors 36 and 37 is connected to the non-inverting input of comparator 38 whose inverting input is grounded. 
     The inverting input of comparator 35 is connected to the tap of a voltage divider formed by resistor 39 to ground and resistor 40 to the negative supply rail 10. 
     The outputs of comparators 35 and 38, connected together, are connected to the inverting input of comparator 41 thru resistor 42 to the test interval buss 16. Comparator 41 has its non-inverting input connected to the junction of resistors 31 and 32, and its output connected thru resistor 43 and light-emitting diode (LED) 44 to ground. 
     FIG. 2 shows a cross-sectional view of a portion of a guitar body 46 fitted with a metal plate 47 having a hole in which audio output jack 23 is mounted. Jack 23 contains switch contact 20 and combined switch and tip contact 21. Attached to jack 23 is circuit board 48 on which are mounted capacitor 24, LED 44, resistor 22 and connector 49. 
     OPERATION OF THE PREFERRED EMBODIMENT 
     Referring once more to FIG. 1; to initiate operation of preamplifier 6, an audio plug (not shown) is inserted into jack 23, causing contacts 20 and 21 to become open-circuited. Resistor 22, having 100k ohms resistance, has negligible shunting effect on the low impedance output of preamplifier 6. Resistor 18, 10 megohms, places a &#34;high&#34; state (near +9 volts) on the on-off buss 17, which makes the output of inverter 25 &#34;low&#34; (near 0 volts), holding capacitor 28 discharged. The on-off buss &#34;high&#34; on one input of NOR gate 29 causes its output and test interval buss 16 to be &#34;low&#34;. This &#34;low&#34; is applied thru resistor 42 to the inverting input of comparator 41 while its non-inverting input is held near +4.5 volts by resistors 31 and 32, each 220k ohms; therefore the output of comparator 41 is an open collector, holding LED 44 off at all times during normal operation of preamplifier 6. 
     Also, during normal operation, the &#34;low&#34; on the test interval buss 16, applied to the inverting input of comparator 30, with approximately +4.5 volts on its non-inverting input from resistors 31 and 32, causes its output to become an open collector, zero-biasing reference circuit 34 to avoid waste current drain during normal operation of preamplifier 6. 
     At turnoff, audio plug removal from jack 23 causes contacts 20 and 21 to close. The resultant &#34;low&#34; at the input of inverter 25 drives its output &#34;high&#34;, initiating charging of capacitor 28 thru resistor 26. Diode 27, being reverse biased, has no effect in this mode. 
     At the instant of turnoff, as the on-off buss 17 goes &#34;low&#34;, both inputs of NOR gate 29 become &#34;low&#34;, driving its output &#34;high&#34;. This &#34;high&#34; on the test interval buss 16 holds the output of NOR gate 15 &#34;low&#34;, delaying the turnoff of supply voltages on rails 5 and 10, while capacitor 28 charges with a time constant of approximately 10 seconds as determined by the capacitance of capacitor 28 (10 uF) and the resistance of resistor 26 (1 megohm). 
     During the test interval, batteries 1 and 7 are both tested as follows: the &#34;high&#34; on the test interval buss 16 enables comparators 35 and 38 thru resistor 42. With the reference voltage at -2.5 volts from reference device 34, the resistance values of resistors 36 and 37, 110k ohms and 220k ohms respectively, cause the threshold of comparator 38 to correspond with +5 volts on supply rail 5. Above +5 volts the output of comparator 38 remains an open circuit. Similarly, the resistance values of resistors 39 and 40, each 220k ohms, cause the threshold of comparator 35 to correspond with -5 volts on rail 10. When the negative voltage on rail 10 exceeds 5 volts, the output of comparator 35 remains an open circuit. 
     When the outputs of comparators 35 and 38 are both open circuits, resistor 42 applies the test interval &#34;high&#34; to the inverting input of comparator 41, driving its output to near -9 volts, holding LED 44 on, indicating that the voltages of batteries 1 and 7 both exceed a threshold level of 5.1 volts, considering the voltage drop across transistor 3 and FET 9 is about 0.1 volt for each. 
     If the voltage of either battery 1 or battery 7 is below 5.1 volts during the test interval, the output of comparator 38 or 35 respectively will &#34;pull down&#34; driving the output of comparator 41 positive, holding LED 44 off, indicating that at least one of the two batteries 1 or 7 has failed the voltage test. 
     The end of the test interval occurs when capacitor 28 charges up to the threshold level of NOR gate 29, approximately 4.5 volts (half the supply voltage). Then NOR gate 29 changes state, driving test interval buss 16 &#34;low&#34;, changing the state of NOR gate 15 to &#34;high&#34; at its output, turning off transistors 3 and 4, and FET 9, thus removing the supply voltages from rails 5 and 10. 
     FIG. 2 shows LED 44 mounted on circuit board 48 which is fastened to jack 23, positioned so that LED 44 is readily observed through the 1/4&#34; opening of jack 23 when the audio plug is removed to turn preamplifier 6 off. Locating the LED in this manner allows this invention to be incorporated into a guitar without altering its external appearance and styling in any way, and also shields the LED from extraneous light. 
     Diode 27 serves to reset the timing circuit by discharging capacitor 28 each time the amplifier is turned on, to ensure the correct test interval timing even after a brief period of operation. 
     Capacitors 2 and 8, each 0.1 uF, serve to bypass batteries 1 and 7 to ensure proper amplifier performance unaffected by battery resistance variations. 
     Capacitor 45 bypasses interference and noise which could otherwise cause erratic turnon or turnoff. 
     The function of FET 9 could be performed by an NPN Darlington transistor, with appropriate selection of resistance values for resistors 11 and 12. 
     Typical component values and types: 
     
         ______________________________________Resistors: 43                680    ohms      33                5.6k   ohms      14                10k    ohms      13, 19            100k   ohms      11, 12, 31, 32, 39, 40, 42                        220k   ohmsNOR gates 15, 29, Inverter 25: MOS IC type 4001(for Inverter 25, both inputs connected together)Comparators 30, 35, 38 and 41: TTL IC type LM3392.5 Volt Reference Device 34: LM336ZTransistors 3, 4: MPSA55FET 9: N-channel type VN10KMJack 23: Switchcraft 112A______________________________________ 
    
     The utility of this invention is applicable to many different types of battery operated appliances, instruments and the like, and the invention is susceptable of numerous variations, alternatives and modifications by those skilled in the art of electronics, without departing from the spirit of the invention. All such variations, alternatives and modifications are intended to fall within the scope of this invention.