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                                                                    EXHIBIT 4.1

                            SECURED PROMISSORY NOTE


Date:  September 3, 1999

Maker:  SIDT, Inc., a Delaware corporation

Maker's Mailing Address:  3006 Longhorn Blvd., Ste. 107, Austin, Texas 78758

Payee:  Sign Builders of America, Inc., a Texas corporation, and Sign Builders,
Inc., a Texas corporation

Place for Payment:  4701 Staggerbrush, Apt. 234, Austin, Texas 78749

Principal Amount:  Four Hundred Fifty Thousand and No/100 ($450,000.00) Dollars

Annual Interest Rate on Unpaid Principal from Date:  Six (6%) percent

Annual Interest Rate on Matured, Unpaid Amounts:  Highest Allowed by Law

Terms of Payment (principal and interest):  This note is due and payable in two
equal installments of $228,325.12. The first installment is due and payable on
March 3, 1999. The second installment is due and payable on September 3, 1999.

Security for Payment:  This note is secured by a Security Agreement of even
date herewith from Maker to Payee granting a security interest in and to all
property described in or referred to in said Security Agreement.

     Maker promises to pay to the order of Payee at the place for payment and
according to the terms of payment the principal amount plus interest at the
rates stated above. All unpaid amounts shall be due by the final scheduled
payment date.

     On default in the payment of this note or in the performance of any
obligation in any instrument securing or collateral to it, the unpaid principal
balance and earned interest on this note shall become immediately due at the
election of Payee. Maker and each




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surety, endorser, and guarantor waive all demands for payment, presentations
for payment, notices of intention to accelerate maturity, notices of
acceleration of maturity, protests, and notices of protest.


     If this note or any instrument securing or collateral to it is given to an
attorney for collection or enforcement, or if suit is brought for collection or
enforcement, or if it is collected or enforced through probate, bankruptcy, or
other judicial proceeding, then Maker shall pay Payee all costs of collection
and enforcement, including reasonable attorney's fees and court costs, in
addition to other amounts due. Reasonable attorney's fees shall be 10% of all
amounts due unless either party pleads otherwise.


     Interest on the debt evidenced by this note shall not exceed the maximum
amount of nonusurious interest that may be contracted for, taken, reserved,
charged, or received under law; any interest in excess of that maximum amount
shall be credited on the principal of the debt or, if that has been paid,
refunded. On any acceleration or required or permitted prepayment, any such
excess shall be canceled automatically as of the acceleration or prepayment or,
if already paid, credited on the principal of the debt or, if the principal of
the debt has been paid, refunded. This provision overrides other provisions in
this and all other instruments concerning the debt.


     Each Maker is responsible for all obligations represented by this note.

     When the text requires, singular nouns and pronouns include the plural.



SIDT, INC.


By: /s/ DOUGLAS P. BAKER
    ------------------------------------------
    Douglas P. Baker
    Vice President and Chief Financial Officer


                                    GUARANTY

     FOR VALUE RECEIVED, the undersigned, SI Diamond Technology, Inc., a Texas
corporation, ("Guarantor") absolutely and unconditionally guarantees payment of
this note according to its






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terms to the same extent as if Guarantor were maker of this note. Guarantor
waives all demand and all notices, including notice of intention to accelerate
the maturity, notice of acceleration of maturity, notice of nonpayment,
presentment for payment, protest, notice of protest, suit, and diligence.
Guarantor also waives any notice of and defense based on the extension of time
of payment or change in methods of payment and consent to all renewals,
extensions, and other adjustments in the manner of payment of this note. This
is a guaranty of payment and performance, not of collection, and it is an
agreement of guaranty, not of suretyship. Guarantor waives all requirements of
law, if any, to require that any action be brought against Maker before
resorting to this guaranty.

     Guarantor hereby agrees that its obligations under the terms of this
guaranty shall not be released, diminished, impaired, reduced, or affected by
the occurrence of any one or more of the following events: (a) the taking or
accepting of any other security or guaranty for any or all of the Guaranteed
Indebtedness; (b) any release, surrender, exchange, subordination, or loss of
any security at any time existing in connection with any or all of the
Guaranteed Indebtedness; (c) any partial release of the liability of Guarantor
hereunder, or, if there is more than one person and/or entity signing this
guaranty, the release of any one or more of them, hereunder; (d) the death,
insolvency, bankruptcy, disability, or lack of corporate power of Maker, any of
the undersigned, or any party at any time liable for the payment of any or all
of the Guaranteed Indebtedness, whether now existing or hereafter occurring;
(e) any renewal, extension and/or rearrangement of the payment of any or all of
the Guaranteed Indebtedness, either with or without notice to or consent of
Guarantor, or any adjustment, indulgence, forbearance, or compromise that may
be granted or given by Payee to Maker of Guarantor; (f) any neglect, delay,
omission, failure, or refusal of Payee to take or prosecute any action for the
collection of any of the Guaranteed Indebtedness or to foreclose or take or
prosecute any action in connection with any instrument or agreement evidencing
or securing all or any part of the Guaranteed Indebtedness; (g) any failure of
Payee to notify Guarantor of any renewal, extension, or assignment of the
Guaranteed Indebtedness or any part thereof, or the release of any security or
of any other action taken or refrained from being taken by Payee against Maker
or any new agreement between Payee and Maker, it being understood that Payee
shall not be required to give





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Guarantor any notice of any kind under any circumstances whatsoever with respect
to or in connection with the Guaranteed Indebtedness; (h) in the event that
Maker is a corporation, joint stock association, or partnership, or is hereafter
incorporated, the unenforceability of all or any part of the Guaranteed
Indebtedness against Maker by reason of the fact that the Guaranteed
Indebtedness exceeds the amount permitted by law, the act or creating the
Guaranteed Indebtedness, or any part thereof, is ultra vires, or the officers
creating same acted in excess of their authority; or (i) any payment by Maker to
Payee is held to constitute a preference under the bankruptcy laws or if for any
other reason Payee is required to refund such payment or pay the amount thereof
to someone else.



     This guaranty is for the benefit of Payee and Payee's representatives,
successors, and assigns, and in the event of an assignment of the Guaranteed
Indebtedness, or any part thereof, the rights and benefits hereunder, to the
extent applicable to the indebtedness so assigned, may be transferred with such
indebtedness. This guaranty is binding not only on Guarantor, but on Guarantor's
representatives, successors, and assigns, and if this guaranty is signed by more
than one person and/or entity then all of the obligations of Guarantor arising
herein shall be jointly and severally binding on each of the undersigned, and
their respective heirs, personal representatives, successors, and assigns.


     Date: September 3, 1999.


SI DIAMOND TECHNOLOGY, INC.


By: /s/ DOUGLAS P. BAKER
    ------------------------------------------
    Douglas P. Baker
    Vice President and Chief Financial Officer



Guarantor's Address:

3006 Longhorn Blvd., Ste. 107
Austin, Texas 78758