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  • AM Radio Receiver Using the NE602 Balanced Mixer"
    slightly toward the correct point without losing the station i e If the station is located at 700 and your tuning dial is pointing higher at 750 slightly move it down toward the correct 700 point more capacitance 6 Readjust T2 T3 and antenna coil for best response 7 Repeat steps 4 5 and 6 until the station is heard loud and clear and no further improvement can be made 8 Remove the wire antenna and readjust the antenna coil T2 and T3 for best response Note the antenna coil should not end up at the center of the loopstick This will indicate not enough inductance and a few more turns of wire may be needed on the antenna coil The optimum position for the coil is near the center slightly offset toward one end If it ends up very near one end of the stick you may want to remove a few turns which will allow the coil to be closer to the center 9 At this point several stations should be heard loud and clear but minor adjustments may be needed to optimize the entire band Select a station near the bottom of the band 600KHz and adjust the antenna coil and oscillator coil for best response Note that only very small adjustments to the red oscillator coil may be needed Then select a station near the top of the band 1500Khz and adjust the 2 trimmer caps on the back of the main tuning capacitor for best response Repeat this process until both ends are optimized Be sure the 2 trimmer caps do not end up fully open or closed If they do note the position and slightly adjust the main capacitor to compensate For example if the trimmers are fully closed adjust the main capacitor slightly lower more capacitance and then readjust the trimmers so the peak occurs somewhere between min and max AM Radio Receiver With Additional IF Stage Pictured above is the same circuit with an additional IF stage added for greater sensitivity Overall gain can be adjusted with the 1K resistors in the emitter leg of the 2N3904 transistors The circuit board was assembled using multiturn 10K pots in place of the 1K resistors and then adjusted for best performance The pots are the 2 little blue items just to the left of the tuning cap I think I ended up with about 750 ohms The emitter bypass caps are not needed since there is plenty of gain available without them The caps two yellow items near the pots are still in the board but not connected I didn t know if they were needed or not so I put them in there anyway and later disconnected them Removing the bypass caps also increases the input impedance so that both IF stages can use the black IF coils which have higher secondary impedances and thus more voltage than the yellow or white coils You might be able to replace the yellow coil with

    Original URL path: http://bowdenshobbycircuits.info/radio.htm (2016-04-26)
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  • Misc
    inductor Menu Whistle On Whistle Off This is an extension of the CMOS toggle flip flop circuit shown in the Circuits controlling relays section with the addition of two bandpass filters and condenser microphone so the relay can be toggled by whistling at it The condender mic used is a PC board mount Radio Shack 270 090C The filters are tuned to about 1700 Hz or the third Ab above middle C on a piano keyboard which is a fairly easy note for me to whistle Resistor values for the filter can be computed using the three formulas below but we need to assume a gain and Q factor for the filter and the Q of the circuit must be greater than the square root of Gain 2 The microphone produces only a couple millivolts so the overall gain needs to be around 4000 or around 65 for each filter The Q or quality factor is the ratio of the center frequency to the bandwidth 3dB points and was chosen to be 8 which is greater than 5 7 which is the minimum value for a gain of 65 Both capacitor values need to be the same for easy computation of the resistor values and were chosen to be 0 01uF which is a common value and usable at audio frequencies From those assumptions the resistor values can be worked out from the following formulas R1 Q G C 2 Pi F 8 65 01 6 6 28 1700 1152 or 1 1K R2 Q 2 Q 2 G C 2 Pi F 8 128 65 01 6 6 28 1700 1189 or 1 2K R3 2 Q C 2 Pi F 16 01 6 6 28 1700 150K The op amps are biased using a voltage divider of two 10K resistors so the output will be centered around half the supply voltage or 6 volts The output of the second filter charges a 1uF cap at the base of a NPN transistor 2N3904 or similar The emitter voltage is biased at 6 6 volts using the 3 3K and 2 7K resistors so that the transistor will conduct and trigger the flip flop when the peak signal from the filter reaches 8 volts The 8 volt figure is the emitter voltage 6 6 plus the emitter base voltage drop 0 7 plus the diode drop 0 7 The sensitivity can be adjusted by changing the value of either the 2 7K or 3 3K resistors so that more or less signal amplitude is needed to trigger the flop flop Menu DC to DC Converter The circuit below is a DC to DC converter using a standard 12 VAC center tapped power transformer wired as a blocking oscillator The circuit is not very efficient but will produce a high voltage usable for low power applications The input battery voltage is raised by a factor of 10 across the transformer and further raised by a voltage tripler consisting of three capacitors

    Original URL path: http://bowdenshobbycircuits.info/page4.htm (2016-04-26)
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  • Op-Amp Basics
    using opamps as active 2nd order filters Three 2nd order filters are shown low pass high pass and bandpass Each of these filters will attenuate frequencies outside their passband at a rate of 12dB per octave or 1 4 the voltage amplitude for each octave of frequency increase or decrease outside the passband First order low or high pass cutoff frequency 3dB point 1 2pi R C 2nd order low or high pass cutoff frequency 3dB point 1 2pi R1 R2 C1 C2 5 Example for 200 Hz cutoff frequency R1 R2 7 95K C1 C2 0 1uF Menu Single Op Amp Bandpass Filter A bandpass filter passes a range of frequencies while rejecting frequencies outside the upper and lower limits of the passband The range of frequencies to be passed is called the passband and extends from a point below the center frequency to a point above the center frequency where the output voltage falls about 70 of the output voltage at the center frequency These two points are not equally spaced above and below the center frequency but will look equally spaced if plotted on a log graph The percentage change from the lower point to the center will be the same as from the center to the upper but not the absolute amount This is similar to a musical keyboard where each key is separated from the next by the same percentage change in frequency but not the absolute amount The filter bandwidth BW is the difference between the upper and lower passband frequencies A formula relating the upper lower and center frequencies of the passband is Center Frequency Square Root of Lower Frequency Upper Frequency The quality factor or Q of the filter is a measure of the distance between the upper and lower frequency points and is defined as Center Frequency BW so that as the passband gets narrower around the same center frequency the Q factor becomes higher The quality factor represents the sharpness of the filter or rate that the amplitude falls as the input frequency moves away from the center frequency during the first octave As the frequency gets more than one octave away from center frequency the rollof approaches 6 dB per octave regardless of Q value Approximate rolloff rates for different Q values for a single octave change from center frequency are Q 1 6 dB Q 5 18 dB Q 10 24 dB Q 50 40 dB For a single op amp bandpass filter with both capacitors the same value the Q factor must be greater than the square root of half the gain so that a gain of 98 would require a Q factor of 7 or more The example below shows a 1700 Hz bandpass filter with a Q of 8 and a gain of 65 at center frequency 1700 Hz Resistor values for the filter can be worked out using the three formulas below Both capacitor values need to be the same for the formulas to

    Original URL path: http://bowdenshobbycircuits.info/opamp.htm (2016-04-26)
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  • 3 Transistor Op-Amp
    would be better Next consideration was the steady state DC current for the differential pair of transistors Q1 Q2 The DC currents of Q1 Q2 should be somewhat balanced with no signal so that both transistors carry near equal DC currents And this current should be somewhat greater than what is needed by Q3 The measured hFE gain of the 2N3906 at 100mA was about 100 so the peak base current of Q3 should be around 1 mA The DC current for Q1 Q2 was then chosen to be about 3 times greater or 3mA for each transistor 6 mA total It could be larger but would increase battery drain Next step was to work out the common emitter resistor R3 Since the base of Q2 is at ground the emitters will be about 700mV and the voltage across R3 will be 3 0 7 2 3 and the value of R3 will be E I 2 3 0 006 390 ohms The total current in R3 will then be 6mA or 3mA for each transistor Q1 and Q2 Next step was to assign a value to the Q3 e b resistor R2 In the balanced no signal condition Q3 s base current will be about 0 5mA and the remaining current for R2 will be 3mA 0 5mA 2 5mA so the resistor value would be R2 Vbe 0015 0 7 0025 280 ohms 270 standard value Resistor R1 will have the same value and is used to measure the DC current in Q1 which should be about 3mA so the voltage across R1 should read around 810mV if things are working right The amplifier gain was selected to be ten or 20dB which defines the ratio of R5 to R6 or 1K and 10K This makes the input

    Original URL path: http://bowdenshobbycircuits.info/3topamp.htm (2016-04-26)
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  • Circuits Page 9
    uF capacitor will charge to about 2 7 volts When the switch is closed the capacitor voltage is applied to the transistor base through a 560 resistor causing the transistor to turn on and activate the relay In the activated state the relay contacts are arranged so the 3 3K resistor and 560 ohm resistor provide a continous current to the transistor base maintaining the activated state While in the activated state the capacitor is allowed to discharge to zero through the 1K resistor When the switch is again closed the capacitor will cause the transistor base to move toward ground deactivating the relay The circuit has three distinct advantages it requires only a few parts always comes up with the relay deactivated and doesn t need any switch debouncing However since the capacitor will begin charging as soon as the button is depressed the button cannot remain depressed too long to avoid re engaging the relay This problem can be minimized with an additional resistor connected from the transistor base to ground so that the base voltage is close to 0 7 volts with the button depressed and the transistor is biased in the linear region With the button held down the relay coil voltage should be somewhere between the pull in and drop out voltages so that the relay will maintain the last toggled state This worked out to about 820 ohms for the circuit I built using a 12 volt 120 ohm relay coil and 2N3053 transistor Temperature changes will effect the situation but the operation is still greatly improved I heated the transistor with a hair dryer and found that the relay will re engage with the button held down for approximately 1 second but this is not much of a problem under normal operation Menu Single MOSFET Relay Toggle Circuit This circuit is similar to the one above but uses a N channel mosfet such as IRF530 540 640 etc in place of the NPN transistor Smaller mosfets could be used but I don t know the part numbers I tested the circuit with a IRF640 IRFZ44 IRFZ34 and REP50N06 The circuit has the same three advantages it requires only a few parts always comes up with the relay deactivated and doesn t need any switch debouncing In operation when the relay is deactivated the 100uF capacitor will charge to 6 volts When the button is pressed the capacitor will apply 6 volts to the MOSFET gate turning it on The capacitor voltage and gate voltage will fall from 6 to 3 volts in about 200 mS which should be enough time for the relay contacts to move For very slow relays a larger capacitor may be needed When the relay energizes the contacts will apply 12 volts to the 3 3K resistor producing 6 volts at the gate which will keep the relay energized indefinetly The capacitor will now discharge to zero since the 12 relay contact is no longer connected to the 15K resistor When the button is again pressed the capacitor will apply zero volts to the gate turning off the relay There should be no problem holding down the button causing the relay to re engage since the gate voltage will be only about 1 8 volts when the button is held down and the mosfet requires about 3 5 volts or more to start conducting But you do need to wait about 1 second or longer between button presses so the capacitor has time to charge or discharge Two push buttons are shown but you could have several more in parallel to control the relay from several different locations Menu CMOS Toggle Flip Flop Using Push Button The circuit below uses a CMOS dual D flip flop CD4013 to toggle a relay or other load with a momentary push button Several push buttons can be wired in parallel to control the relay from multiple locations A high level from the push button is coupled to the set line through a small 0 1uF capacitor The high level from the Q output is inverted by the upper transistor and supplies a low reset level to the reset line for about 400 mS after which time the reset line returns to a high state and resets the flip flop The lower flip flop section is configured for toggle operation and changes state on the rising edge of the clock line or at the same time as the upper flip flop moves to the set condition The switch is debounced due to the short duration of the set signal relative to the long duration before the circuit is reset The Q or Qbar outputs will only supply about 2 mA of current so a buffer transistor or power MOSFET is needed to drive a relay coil or lamp or other load A 2N3904 or most any small signal NPN transistor can be used for relay coil resistances of 250 ohms or more A 2N3053 or medium power 500 mA transistor should be used for coil resistances below 250 ohms The 47 ohm resistor and 10uF capacitor serve to decouple the circuit from the power supply and filter out any short duration noise signals that may be present The RC network 1 47K at the SET line pin 8 serves as a power on reset to ensure the relay is denergized when circuit power is first applied The reset idea was suggested by Terry Pinnell who used the circuit to control a shed light from multiple locations Menu CMOS Toggle Flip Flop Using Laser Pointer The circuit below is similar to the one above but can be used with a laser pointer to toggle the relay rather than a push button The IR photo transistor Q1 Radio Shack 276 145A or similar is connected to the set input pin 6 The photo transistor should be shielded from direct light so that the voltage at the set input pin 6 is less than 1 volt

    Original URL path: http://bowdenshobbycircuits.info/page9.htm (2016-04-26)
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  • Buck Converter.
    an oscillator with 31 duty cycle at about 11 5 Khz or 66us off time and 21uS on time for the MOSFET switch The remaining 4 inverters are used in parallel to provide additional drive current to the gate of the MOSFET The duty cycle can be adjusted with either the 15K or 20K resistors The minimum inductor value was worked out from E L di dt and a LED current of 250mA The minimum value is where the current falls to 0 during the switch off time or 66uS The peak inductor current would then be twice the average or 500mA and the inductor will charge from 0 to 500mA in 21uS So di dt is 0 5 000021 23810 amps per second The inductor voltage E will be 12 minus the load voltage 3 7 or 8 3 volts and the minimum inductor value L will be 8 3 23810 0 35 mH The actual value used should be somewhat higher to avoid the current falling to zero and to avoid large peak currents and possible saturation The example here uses a approximate 2 mH inductor so the change in current is about 100mA and the peak current is lower at about 300mA The current waveform is shown in the LTspice picture below Notice the current ramps from about 50mA below the average current to about 50mA above the average or about 100mA total change The 15 ohm resistor in the LTspice picture represents the LED plus a 2 2 ohm resistor The MOSFET is represented by the SW switch component and the drive circuit by the V3 symbol The inductor pictured below should be rated for saturation current of more than the peak current or maybe 300mA in this case The toroid inductor used is fairly large

    Original URL path: http://bowdenshobbycircuits.info/buck.htm (2016-04-26)
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  • Solar Cell Boost Converter.
    edges and a few tiny cracks but still perform well and sell at discount There are also many good deals on ebay The 10Khz oscillator and drive circuit obtain power from the battery under charge which should be grater than 4 volts The output stage mosfet and inductor obtain power from the solar array and produce a charging current through the schotty diode VSK 330 Efficiency is improved with 220uF capacitors added across the input and output A 12 volt zener diode and 120 ohm resistor were added to protect the circuit from excessive voltage in the event the battery is disconnected during operation Additional protection is obtained with the TL431 voltage reference diode which limits the output voltage to 18 volts If the output exceeds 18 volts the cathode of the TL431 falls stopping the oscillator until the output falls below 16 volts In operation the duty cycle of the switching waveform is adjusted with the 100K pot for maximum current into the battery This adjustment can be made by monitoring the voltage across the 1 5 ohm resistor and adjusting for maximum voltage This should be the optium setting where efficiency is highest and maximum power is extracted

    Original URL path: http://bowdenshobbycircuits.info/boost.htm (2016-04-26)
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  • Game Show, Chaser, Charger, Derby, Neons
    to the desired current The circuit lower right illustrates using a LM317 variable voltage regulator as a constant current source The voltage between the adjustment terminal and the output terminal is always 1 25 volts so by connecting the adjustment terminal to the load and placing a resistor R between the load and the output terminal a constant current of 1 25 R is established Thus we need a 12 ohm resistor R to get 100mA of charge current and a 1 2 ohm 2 watt resistor for 1 amp of current A diode is used in series with the input to prevent the batteries from applying a reverse voltage to the regulator if the power is turned off while the batteries are still connected It s probably a good idea to remove the batteries before turning off the power Menu 120VAC Lamp Chaser This circuit is basically the same as the 10 channel LED sequencer with the addition of solid state relays to control the AC lamps The relay shown in the diagram is a Radio Shack 3 amp unit part no 275 310 that requires 1 2 volts DC to activate No current spec was given but I assume it needs just a few milliamps to light the internal LED A 360 ohm resistor is shown which would limit the current to 17 mA using a 9 volt supply I tested the circuit using a solid state relay of unknown type which required only 1 5 mA at 3 volts but operates up to 30 volts DC and a much higher current The chaser circuit can be expanded up to 10 channels with additional relays and driver transistors The 4017 decade counter reset line pin 15 is connected to the fifth count pin 10 so that the lamps sequence from 1 to 4 and then repeat For additional stages the reset pin would be connected to a higher count Menu Game Show Indicator Lights Who s First The circuit below turns on a light corresponding to the first of several buttons pressed in a Who s First game Three stages are shown but the circuit can be extended to include any number of buttons and lamps Three SCRs silicon controlled rectifiers are connected with a common cathode resistor 50 ohm so that when any SCR conducts the voltage on the cathodes will rise about 7 volts above the voltage at the junction of the 51K and 1K ohm resistors and prevent triggering of a second SCR When all lamps are off and a button is pressed the corresponding SCR is triggered due to the voltage at the divider junction being higher than the cathode Once triggered the SCR will remain conducting until current is interrupted by the reset switch Or you can just turn the power off and back on A 50 ohm 5 watt resistor was selected to produce a 10 volt drop at 200 mA when a single 25 watt lamp comes on Higher wattage lamps

    Original URL path: http://bowdenshobbycircuits.info/page7.htm (2016-04-26)
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