Control circuit for a tone generator

A control circuit for a tone generator that is adapted to cause the tone generator to produce a plurality of readily distinguishable audible sounds, including a chime sound, a pulse tone and a steady tone. The control circuit has particular application for use on an automobile to monitor such conditions as "seat belts buckled", "headlamps on", and "key in the ignition". The control circuit includes three oscillator circuits that are adapted to produce square wave output signals at three different frequencies. The chime sound is generated by pulse width modulating the highest frequency signal and "restriking" the decaying sound at the frequency of the lowest frequency oscillator. The steady tone is produced by driving the tone generator directly with the output signal from the highest frequency oscillator. To generate the pulse tone, the highest frequency oscillator is repeatedly enabled and disabled in accordance with the output signal from the middle frequency oscillator. Novel pulse width modulating circuitry for producing the chime sound is also disclosed.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a control circuit for a tone generator and 
in particular to a control circuit that is adapted to cause the tone 
generator to produce a plurality of perceptively distinguishable audible 
sounds, including a chime sound, a pulse tone and a steady tone. 
Current federal regulations require that all automobiles be equipped with 
devices that will provide a four to eight second audible warning whenever 
the automobile is attempted to be operated without the seat belts properly 
fastened. Such devices typically take the form of a buzzer unit that is 
controlled by a bimetallic timer circuit which is adjusted to time out 
within the prescribed time period. In addition, the same buzzer unit is 
typically utilized to also provide an audible signal to indicate other 
monitored conditions, such as when the keys are left in the ignition or 
when the headlamps are left on after the ignition is turned off. 
Devices of this type have two basic disadvantages. Not only is the sound 
produced by such buzzer units unpleasant and often irritating, but the 
same audible sound is produced for all three warning conditions. Hence, it 
is often not immediately discernable by the operator of the vehicle which 
of the monitored conditions is triggering the alarm. 
Thus, it is one of the principal objects of the present invention to 
provide an improved control circuit that is adapted to cause the tone 
generator to which it is connected to produce readily distinguishable 
audible signals for each of the conditions monitored. In particular, the 
present control circuit is adapted to drive a tone generator to produce a 
chime sound, a pulse tone, or a steady tone, depending upon the particular 
enabling signal received. Thus, it can readily be seen that the present 
invention is particularly suited for use in an automobile to control an 
audible warning device that is utilized to monitor the three conditions 
noted, namely, the seat belts, the headlamps, and the ignition key. 
Moreover, it will also be appreciated from the following description of 
the preferred embodiment herein that the present invention is adapted to 
drive the tone generator so that the sounds produced are pleasant to the 
ear. 
In addition, it will also be seen that the control circuit of the present 
invention is reliable, relatively simple in design, and therefore 
exceptionally low in cost. Moreover, the size of the circuit board 
required for the control circuit is small, thus making the control circuit 
further suitable for automotive use where space is at a premium. 
In general, the control circuit for the present invention comprises three 
oscillator circuits: a high frequency oscillator, a low frequency 
oscillator, and a middle frequency oscillator. The oscillators comprise 
simple logic gate circuits that are adapted to produce square wave output 
signals. The high frequency oscillator and the low frequency oscillator 
are utilized to produce the chime sound. In particular, the high frequency 
oscillator establishes the tone of the chime and the low frequency 
oscillator provides the "striking" rate of the chime. As will subsequently 
be described in greater detail, the high frequency oscillator signal is 
pulse width modulated in a novel manner to produce the desired decay 
effect in the amplitude of the sound. 
The steady tone is produced simply by driving the tone generator with the 
output signal from the high frequency oscillator. The pulse tone, on the 
other hand, is generated by enabling the middle frequency oscillator and 
utilizing the output signal therefrom to control the enabling of the high 
frequency oscillator. Thus, the tone produced for the steady tone is 
pulsed at the rate of the output signal from the middle frequency 
oscillator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a circuit diagram of a control circuit 10 according to 
the present invention is shown. Basically, the present control circuit 10 
comprises three oscillator circuits 12-16, a modulator circuit 18 for 
producing the chime effect, and a timing circuit 20 for providing the four 
to eight second logic signal utilized to provide the prescribed seat belt 
warning signal. The control circuit 10 is adapted to drive a tone 
generator, represented by coil L1 and resistor R19, preferably of the type 
disclosed in the copending U.S. application of Irvin Rea et al., entitled 
"Tone Generator", Ser. No. 917,174, filed June 20, 1978, and assigned to 
the assignee of the present invention. 
The timing circuit 20 is basically comprised of a triple PTC element 25, as 
well as resistors R3 and R6. The triple PTC element 25 utilized in the 
preferred embodiment herein is of the type described in the copending U.S. 
application of Michael Slavin et al., entitled "Tone Generator and Control 
Circuit Therefor", Ser. No. 814,417, filed July 11, 1977, also assigned to 
the assignee of the present invention. The timing circuit 20 is adapted to 
provide a HI logic signal on line 35 for the initial four to eight second 
time period after the ignition has been turned on. In addition, the timing 
circuit 20 is further adapted to provide a coincidentally timed signal on 
line 27 which is connected to a seat belt warning lamp located on the 
dashboard of the automobile. 
The high frequency oscillator 12 comprises a pair of logic gates, herein 
NOR-gates 22 and 24, a capacitor C3 and resistors R9, R13 and R14. The 
values of the circuit components, and in particular capacitor C3, are 
selected so that oscillator 12 will produce a square wave output signal at 
node 13 at a frequency of approximately 750 Hz. Diode D1 is included in 
the oscillator circuit 12 to provide a greater than 50 percent duty cycle 
in the oscillator output signal. 
The low frequency oscillator circuit 14 similarly comprises a pair of logic 
gates, herein NOR-gates 26 and 28, a capacitor C2 and resistors R11 and 
R12. The values of the circuit components, and in particular capacitor C2, 
are selected to provide a square wave output signal at node 15 having a 
frequency of approximately 0.75 Hz. Diode D2 is included in the oscillator 
circuit 14 to also produce a greater than 50 percent duty cycle in the 
output signal from oscillator 14. 
Oscillator circuit 16 likewise comprises a pair of logic gates, herein 
NOR-gates 30 and 32, a capacitor C1 and a resistor R10. The values of 
capacitor C1 and resistor R10 are selected so that oscillator 16 produces 
a square wave output signal at node 17 at a frequency of approximately 1.5 
Hz. Since a diode is not included in oscillator circuit 16, the output 
signal of the oscillator 16 has a 50 percent duty cycle. 
As noted previously, the control circuit 10 of the present invention is 
adapted to produce three readily distinguishable sounds; i.e., a chime, a 
pulse tone, and a steady tone. As will subsequently be seen, the chime 
sound is produced whenever the signal on line 46 from the seat belt 
terminal is LO and the logic signal from the output of the timing circuit 
20 on line 35 is HI. In addition, the pulse tone is produced whenever the 
signal on line 50 connected to the ignition terminal is LO and the signal 
on line 52 connected to the headlamp terminal is HI. Finally, the steady 
tone is produced whenever the signal on line 54 connected to key terminal 
is LO. 
Whenever the ignition is initially turned on, the timed HI logic signal on 
line 35 causes a LO signal to be produced on line 48 from the output of 
inverter 36. However, the output of NOR-gate 34 will not go HI unless the 
signal on line 46 is also LO, indicating that the seat belts are not 
properly fastened. When this occurs, the HI signal produced at the output 
of NOR-gate 34 results in a LO signal at the output of NOR-gate 38, which 
enables the low frequency oscillator 14. In addition, the LO signal at the 
output of NOR-gate 38 is provided through an inverter 40 to one of the 
inputs of NOR-gate 32, thereby disabling oscillator circuit 16. Further, 
the resulting LO signal produced at the output of NOR-gate 32 is provided 
through resistor R7 to line 54, thereby enabling the high frequency 
oscillator 12. Thus, to reiterate, a HI signal at the output of NOR-gate 
34 enables the low frequency and high frequency oscillator circuits 12 and 
14 and disables the middle frequency oscillator circuit 16. 
The square wave output signal from the high frequency oscillator circuit 12 
is provided to a novel modulator circuit 18 which includes a pair of decay 
circuits. The first decay circuit is comprised of capacitor C4 and 
resistors R16 and R18. The second decay circuit is comprised of capacitor 
C5 and resistor R18. In the preferred embodiment herein, the value of 
capacitor C5 is approximately 100 times larger than the value of capacitor 
C4, hence the time constant associated with the first decay circuit is 
approximately 100 times faster than the time constant associated with the 
second decay circuit. The connection between capacitor C4 and the input of 
NAND-gate 42 essentially provides the electrical equivalent of a diode D3, 
shown in phantom between capacitor C4 and ground. 
The application of the square wave output signal at node 13 from the high 
frequency oscillator 12 through capacitor C4 results in the modified 
waveform at node 56 shown in FIG. 2, with the slope of the declining 
portion of the waveform being determined by the discharge rate of 
capacitor C4, through resistor R18 and transistor Q2 to ground. The charge 
on capacitor C4 is principally discharged through resistor R18 and 
transistor Q2 to ground, rather than through resistor R16 to ground, due 
to the fact that the value of resistor R16 is substantially larger than 
the value of resistor R18. The sole purpose of resistor R16 is thus to 
ensure that transistor Q2 turns completely off between pulses. 
When the low frequency oscillator circuit 14 is enabled, the output signal 
therefrom at node 15 is normally HI, due to the large percentage duty 
cycle of the signal. Hence, transistor Q1, which has its base connected 
through resistor R17 to the output of oscillator 14, is normally off. As a 
result, the signal at node 56 is further modified by the substantially 
slower time constant of the second decay circuit as capacitor C5 
"discharges" through resistor R18 and transistor Q2 to ground. In 
actuality, capacitor C5 is initially discharged in that both sides of the 
capacitor are at B+, and is thereafter charged negatively with respect to 
ground. However, because it is less confusing if a capacitor is considered 
to be charging when the potential is going up and discharging when the 
potential is going down, the condition of capacitor C5 will be described 
in this manner. Accordingly, it will be appreciated that the signal 
appearing at node 58 will correspond to that shown in FIG. 2, which is 
essentially the same as the waveform appearing at node 56 gradually 
declining at the "discharge" rate of capacitor C5. Thus, as can readily be 
seen from the timing diagram in FIG. 2, as the waveform at node 58 
gradually declines, a lesser and lesser portion of the waveform will 
exceed the turn on threshold of the transistor Q2. Hence, the percentage 
on-time of the transistor Q2 will gradually decline thereby resulting in a 
corresponding decline in the duty cycle of the output signal from 
transistor Q2. Thus, the amplitude of the tone produced by the tone 
generator will also gradually diminish. 
As noted previously, transistor Q1 is normally off when the low frequency 
oscillator 14 is enabled, as a result of the Hi signal on line 60. The 
output of the low frequency oscillator 14 at node 15 will, however, 
briefly go LO approximately every 1.25 seconds. When this occurs, 
transistor Q1 will be momentarily turned on, thereby permitting capacitor 
C5 to "recharge" to B+ potential. The width of the LO pulse from the 
output of the low frequency oscillator 14 is approximately equivalent to 
the duration of seven high frequency pulses to ensure that the capacitor 
C5 will be fully "charged" while transistor Q1 is on. Thus, it will be 
appreciated that the amplitude of the tone produced by the tone generator 
will repeatedly slowly diminish and then "restrike" at the frequency of 
the low frequency oscillator 14. 
When the low frequency oscillator circuit 14 is disabled, the output 
thereof at node 15 is LO, therefore transistor Q1 is turned on. Under such 
circumstances, capacitor C5 is effectively removed from the circuit. As a 
result, the signal appearing at node 56 will be provided directly through 
resistor R18 to the base of transistor Q2. Thus, it can be seen that the 
steady tone is produced merely by providing a LO signal on line 54 to 
enable the high frequency oscillator circuit 12. A LO signal is provided 
on line 54 whenever the door of the vehicle is opened while the key is in 
the ignition. Since the turn-on threshold of transistor Q2 is relatively 
low, the transistor effectively "sees" a square wave signal at its base. 
Hence, the tone generator will produce a steady tone determined by the 
frequency of the high frequency oscillator 12. 
When the headlamps are left on after the ignition has been turned off, a HI 
signal will appear on line 52 and a LO signal will appear on line 50. The 
LO signal on line 50 is inverted by NAND-gate 42, thus resulting in both 
inputs to NAND-gate 44 being HI. The output of NAND-gate 44 will 
accordingly go LO, thereby enabling the mid-frequency oscillator 16. The 
square wave output signal from oscillator 16 at node 17 is provided 
through resistor R7 to the enable line 54 of the high frequency oscillator 
12. The high frequency oscillator 12, therefore, will be repeatedly 
enabled and disabled at the frequency of oscillator 16. Thus, it will be 
appreciated that the tone generator will produce a pulse tone with the 
frequency of the tone being determined by the frequency of the high 
frequency oscillator 12 and the rate of the tone being determined by the 
frequency of oscillator 16. 
While the above description constitutes the preferred embobiment of the 
invention, it will be appreciated that the invention is susceptible to 
modification, variation and change without departing from the proper scope 
or fair meaning of the accompanying claims.