Circuit review needed

Hi guys,

I think I previously put this request in the wrong forum so I replace it here. Basically I found this circuit on the web with an added description of how it works. However, being very new to electronics I can't understand exactly how the two back to back zener diodes come into the equation to make the bi-color diode flash. The bi-color diodes I own have three leads, A1 + A2 + common cathode but this circuit shows only two. Can anyone explain this and maybe confirm if this circuit will operate as described in the text. Here's a link to a web page with the circuit and details if my schemetic doesn't appear.

h**p://www.williamhowarth.esmartweb.com/Circuit.html

Thanks in advanced

John

The first section (U1B) of the LM358 dual op-amp is a 54 (2 x 27 Volts peak per cycle) volt peak-to-peak square wave oscillator. The second section (U1A) reverses polarity and provides ±27 Volts DC output of low impedance. This delivers a Bi-Phasic, sharp rise--time output of ~4 Hz (not critical) for the biological cotton--covered stainless-steel (or gold-plated) electrodes saturated with salt water before applying. Sharp rise--time is considered necessary to provide higher odd harmonics to the stimulus, although "rounded" waveforms will feel different.

The third section is a current--limited 27 Volts DC output from a separate RCA (or 2.5mm) jack for rapid generation of excellent ionic/colloidal silver in water. A three minute cycle in 8 Oz. of room--temperature water makes a ~3-5 PPM concentration.

Op-amp section U1B's 4 Hz oscillator frequency is set by C1 (0.1 uF) and R1 (2.4 meg Ohm). It is configured as a comparator with hysteresis determined by R2 (150 k Ohm). Charging and discharging of C1 is done by the 180º out-of-phase signal through R1. R3 and R4 provide a set--point 1/2 the V+ to the comparator. This insures a 50% duty cycle square wave with an amplitude of slightly less than the ~27 Volt supply.

U1A, the second comparator, is used to invert the output of oscillator U1B. A ~54 Volt peak-to-peak signal will be generated between the op-amps due to their outputs being 180º out-of-phase. U1A's current is limited by potentiometer R5 (100 k Ohm) and R7 (820 Ohm) and is set to individual user's comfort.

The power indicator circuit consists of a bicolor (Red-Green) LED (CR1) and the series combination of two 18 Volt Zener diodes, D1 & D2, with power limited by C2 (22 uF, 35 Volt). This section of the device is automatically disabled when the 3.5 mm plug is inserted into its jack. Therefore the LEDs flash only when batteries sum is over ~21 Volts. If LEDs are dim or extinguished, replace with three fresh 9 Volt Alkaline batteries. C2 used as a limiter allows the LED to flicker on at 1/8 second intervals only as the square wave output reverses polarity

The diode model used in the diagram probably has diodes across the LEDs so that the current in each direction is bypassed around one or the other of the LEDs.

Here is a verbal description. Imagine just one LED. There is an ordinary diode in parallel with it that will conduct in the opposite direction of current compared to the LED. When current goes one direction, it flows through the LED and light comes out. The parallel ordinary diode is reverse biased by 1.5 V or so. When current flows the other direction it goes through the ordinary diode and the LED is reverse biased by 0.7 V or so.

Then you have two of these circuits with different colour LEDs. You place them in series pointing in opposite directions. You should be able to make this circuit with your three lead device if the LEDs are inside pointing towards or away from the common lead.

This reminds me of a trick to do to beginners. You have two light sockets wired in series and each one has a concealed diode wired across the external contacts. (Make sure they are pointing in opposite directions or the line will burn them out.) You put 110 V bulbs in each socket and put 220 V across the whole circuit. Both bulbs glow at full brilliance. Then you unscrew one bulb and the other stays on at full brilliance. For safety you can use lower voltage bulbs such as 6 V pilot lights and a 12 V source.

Thanks a million for your speedy reply. I get the drift by using diodes across the LED's. Will the LM358 dual op-Amp operate as an oscillator in the configuration shown and as described in the authors supplied text? I can't seem to get any oscilloscope output when simulating the schemetic in my "MultiSim 8" software on my PC

Thanks again,

John