Light sensitive control for toaster

In a toaster, a system is constructed to vary the duration of the toasting cycle. An RC network provides an oscillating signal to a counter, which upon the required number of oscillations being counted, triggers a transistor switch. The switch energizes the main solenoid switch which in turn deactivates the heating elements of the toaster. The RC network of the system comprises a variable resistor which may be manually operated, a heat sensitive resistor, and a light sensitive resistor.

BACKGROUND OF THE INVENTION 
Most toasters are now electronically controlled and provide a more 
consistently toasted product then in the past. There are however still 
some significant problems in providing a sensitive control circuit which 
will provide the same degree of toasting on a repetitive basis. When a 
toaster is used for multiple toasting cycles with pauses in between of 
varying duration, the heat retention in the toaster will make the toast 
darker on subsequent cycles of use, forcing a continuous adjustment of the 
toaster. 
The basic circuit of the control system used to adjust the toasting time 
consists of a power supply, an oscillator, a counter, and a transistor 
switch. This is used to activate an on/off solenoid switch which controls 
the power to the toaster heating elements. Adjustment is accomplished by 
varying the time constant of the oscillator. The oscillator is generally 
an RC network having a manually variable resistor. By raising the 
frequency of the oscillator the time in which the counter will respond and 
trigger the transistor switch is reduced. 
A control system of the type designed to compensate for this situation is 
shown in U.S. Pat. No. 5,402,708. This system utilizes a heat sensitive 
resistor to monitor and react to the residual heat accumulated by 
repetitive toasting cycles. A thermistor is used which performs its task 
by reduction of its resistance as the heat increases. Since this is the 
resistor which forms part of the RC network, a reduced period for the 
oscillator results. This translates into a shorter toasting cycle. 
The control system of the '708 patent is still subject to variations in the 
voltage to the heater elements and may not maintain toast color over 
continuous cycling. It also tends to overcompensate. It is the purpose of 
this invention to provide an improved control system for a toaster which 
is more sensitive to repetitive cycling and also to variations in the line 
voltage and wattage. 
SUMMARY OF THE INVENTION 
In a toaster, a system is constructed to vary the duration of the toasting 
cycle. An RC network provides an oscillating signal to a counter, which 
upon the required number of oscillations being counted, triggers a 
transistor switch. The switch energizes the main solenoid switch which in 
turn deactivates the heating elements of the toaster. The RC network of 
the system comprises a variable resistor which may be manually operated, a 
heat sensitive resistor, and a light sensitive resistor. The series of 
resistors control the time constant of the oscillator and allow it to be 
varied in response to diverse stimulus. The heat sensitive resistor is 
positioned to respond to the temperature of the heater element cavity. A 
high temperature glass tube is used to transmit light from the heating 
elements to the light sensitive resistor. In this manner an extremely 
sensitive control is provided which automatically compensates for 
repetitive cycling even with varying pauses between uses and also 
variations in the line voltage to the heating elements.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, the toaster 1 of this invention is of conventional 
shape and is constructed with slots 2 and 3 for the insertion of bread or 
other pastry to be toasted. Calrod type heating elements 4 are mounted on 
either side of the slots 2 and 3 to heat and brown the toast. The workings 
and toast compartment 24 of the toaster 1 are contained in a housing 7 and 
an end cap 8 is removably secured to the housing 7 to enclose the control 
mechanism 5. A manual adjustment 6 is provided in operative connection to 
the control mechanism 5. The control mechanism 5 may include any 
conventional mechanical assembly for raising and lowering the toast and is 
operatively connected to the control system 10. A high temperature glass 
pipe is snaked through the housing adjacent to one of the calrod heaters 4 
to receive light emitted from the heater element 4. The intensity of the 
light will reflect the level of voltage and wattage and also the presence 
of residual heat. The glass pipe 9 transmits the light from the heater 
element 4 to the light sensitive resistor 17 of oscillator 12. 
Referring to FIG. 2, the control system 10 consists of a power supply 
circuit 11, an oscillator 12, a counter 13 and a transistor switching 
circuit 14. Switching circuit 14 directly controls the operation of a 
solenoid switch 15 which is associated with the main mechanical switch 22. 
The activation of the solenoid switch 15 applies the full supply voltage 
to the heater elements, generally 120 volts. 
The purpose of the supply circuit 11 is to provide a low voltage, half wave 
rectified power to the control circuit and may consist of a diode 
rectifier connected in association with a dropping resistor which lowers 
the input voltage to approximately 12 volts. Other components are 
conventionally used to insure a stable power supply for the control 
system. 
Oscillator 12 is shown in FIG. 3 and consists of an RC network including 
variable resistor 19, resistors 21 and 23, capacitor 20, light sensitive 
resistor 17 and heat sensitive resistor 18. It can be seen that changing 
the resistance of any of the resistors of the RC circuit will adjust the 
time constant of the RC network and the frequency of the oscillator 12. 
Variable resistor 19 provides the manual adjustment capability of toaster 
1 and generally is a potentiometer which is varied in resistance by the 
rotation of an external knob 6 as shown in FIG. 1. The operator of the 
toaster sets the knob 6 at what he or she perceives as the proper time for 
toast of the desired color. If no other control was provided toasting 
would be a trial and error process and would vary considerably from cycle 
to cycle. Two additional elements are provided which react to the 
functioning of the toaster by sensing the heat within the toast 
compartment 24 and the intensity of light being emitted from the heating 
elements 4. 
Heat sensitive resistor 18 is positioned to receive heat radiated from the 
toast compartment 24. The resistance of resistor 18 will decrease as the 
heat to which it is exposed increases. Since the frequency of the 
oscillator 12 is inversely proportional to the resistance of the network, 
the frequency goes up as the resistance goes down. This results in the 
shortening of the time in which the heater elements are energized as the 
accumulated heat in the toast compartment 24 increases. 
Light sensitive resistor 17 receives light transmitted through the glass 
pipe 9. The resistance of the resistor 18 decreases as the light intensity 
to which it is exposed increases. Similarly to the resistor 18, this 
results in the shortening of the time in which the heater elements 4 are 
energized as the brightness of the light discharged by the heater element 
increases. 
Capacitor 20 is selected to minimize the effects of heat on the capacitance 
of the RC network. In particular a capacitor having a characteristic 
temperature curve as shown in FIG. 4 is used, for example, a Philips. 
Components monolithic ceramic capacitor model no. X7R. This aids in the 
stabilization of the RC network over the temperature operation range and 
avoids counter productive variations in capacitance which may negate the 
effectiveness of the resistor compensation. 
Counter 13 is a conventional integrated circuit, for example model CD4060B 
manufactured by National Semiconductor, and serves to count a 
predetermined number of cycles of the oscillator 12. When the desired 
number of cycles are complete, the counter 13 will signal transistor 
switching circuit 14. The counter signal causes the transistor switch to 
conduct and energize the solenoid switch 15. This will shut off the heater 
elements 4 and end the toasting cycle. 
The adjustment and compensating resistances must be balanced so as to 
prevent an over compensation occurring because of the response of any one 
component. With this in mind, a heat sensitive resistor 18 having a 
resistance of 100,000 ohms is selected in association with a light 
sensitive resistor 17 that operates in a range of 4000 to 1100 ohms. The 
manual adjustment potentiometer 19 is selected having a range of operation 
of 0 to 100,000 ohms. Capacitor 20 is selected having a capacitance of 
0.22 microfarads. 
In operation the toast is inserted into the toaster slots 2 and 3 and the 
knob 6 is manually adjusted to the desired toasting cycle duration thereby 
energizing the heating elements 4 to begin toasting. The initial cycle 
will generally be the longest, running in the vicinity of 4minutes, 
because there is no accumulated heat. The challenge is to repeat the same 
performance in subsequent operations. A second user then immediately 
inserts a second pair of bread slices into the toaster and initiates 
operation of the heating elements 4. At this point, there is considerable 
residual heat in the toast compartment 24, so a second 4 minute cycle will 
burn or over cook the toast. Partial compensation will be accomplished by 
the heat sensitive resistor 18, but by itself this may create an over 
compensation which results in very light, under done toast. It is desired 
to reduce the subsequent cycle to approximately 2 to 3 minutes. The 
heating elements 4 will obtain a higher temperature and a brighter light 
signature because of the increased heat in the toast compartment 24. The 
light sensitive resistor 17 will respond by reducing its resistance and a 
further increase in the frequency of the oscillator 12 will occur. Further 
cycles will be accelerated at a lesser rate and a reliable toasting 
performance results. 
In this manner compensation is provided for accumulated heat in the toast 
compartment and for variations in the line voltage.