<PAGE>   1
                                                                    EXHIBIT 4.15



                            PATENT SECURITY AGREEMENT


         WHEREAS, TIDEL ENGINEERING, L.P., a Delaware limited partnership
("Grantor"), owns the patents, patent registrations, and patent applications
listed on Schedule 1 annexed hereto, and is a party to, or has been assigned the
rights by the party to, the patent licenses listed on Schedule 1 annexed hereto;
and

         WHEREAS, Grantor, Chase Bank of Texas, N.A., a national banking
association ("Grantee"), and Tidel Technologies, Inc., a Delaware corporation,
are parties to that certain Credit Agreement dated as of the date hereof (as
heretofore or hereafter amended, modified and in effect from time to time, the
"Loan Agreement"), providing for extensions of credit to be made by Grantee to
Grantor; and

         WHEREAS, pursuant to the terms of the Security Agreement (as defined in
the Loan Agreement), Grantor has granted to Grantee a security interest in all
of the assets of Grantor including all right, title and interest of Grantor in,
to and under all now owned and hereafter acquired patents and patent
applications, and all products and proceeds thereof, to secure the payment of
all amounts owing by Grantor under the Loan Agreement including, without
limitation, all the Obligations (as defined in the Loan Agreement);

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Grantor does hereby grant to
Grantee a continuing security interest in all of Grantor's right, title and
interest in, to and under the following (all of the following items or types of
property being herein collectively referred to as the "Patent Collateral"),
whether presently existing or hereafter created or acquired:

                  (1) each patent, patent registration and patent application,
                  including, without limitation, the patents, patent
                  registrations (together with any reissues, continuations or
                  extensions thereof) and patent applications referred to in
                  Schedule 1 annexed hereto;

                  (2) each patent license, including, without limitation, each
                  patent license listed on Schedule 1 annexed hereto; and

                  (3) all products and proceeds of the foregoing, including,
                  without limitation, any claim by Grantor against third parties
                  for past, present or future infringement of any patent or
                  patent registration including, without limitation, the patents
                  and patent registrations referred to in Schedule 1 annexed
                  hereto, the patent registrations issued with respect to the
                  patent applications referred in Schedule 1 and the patents
                  licensed under any patent license.

This security interest is granted in conjunction with the security interests
granted to Grantee pursuant to the Loan Agreement. Grantor hereby acknowledges
and affirms that the rights and remedies of Grantee with respect to the security
interest in the Patent Collateral made and granted hereby are



PATENT SECURITY AGREEMENT - Page 1

<PAGE>   2

more fully set forth in the Loan Agreement, the terms and provisions of which
are incorporated by reference herein as if fully set forth herein.

         This Patent Security Agreement is given in renewal, amendment,
replacement, and restatement in its entirety (but not in novation,
extinguishment or satisfaction) of that certain Patent Security Agreement dated
June 12, 1997, executed by Tidel Engineering, Inc., for the benefit of Grantee,
successor-in-interest to Texas Commerce Bank National Association (the "Prior
Agreement"). To the extent of any conflict between the terms of this Patent
Security Agreement and the terms of the Prior Agreement, the terms of this
Patent Security Agreement shall control.

         IN WITNESS WHEREOF, Grantor has caused this Patent Security Agreement
to be duly executed by its duly authorized officer thereunto as of the 1st day
of April, 1999.

                               GRANTOR:

                               TIDEL ENGINEERING, L.P.,
                               a Delaware limited partnership

                               By: Tidel Cash Systems, Inc., its general partner


                                   By:
                                      ------------------------------------------
                                      Mark K. Levenick, President and Chief
                                         Executive Officer

Acknowledged, agreed and accepted as
of the date hereof:

GRANTEE:

CHASE BANK OF TEXAS, N.A.



By:
   -----------------------------------
   Joanne Bramanti,
   Vice President




PATENT SECURITY AGREEMENT - Page 2

<PAGE>   3

                                 ACKNOWLEDGMENT


STATE OF TEXAS     )
                   )       ss.
COUNTY OF DALLAS   )

         On the _____ day of _______, 1999 before me personally appeared Mark K.
Levenick, to me personally known or proved to me on the basis of satisfactory
evidence to be the person described in and who executed the foregoing instrument
as the President and Chief Executive Officer of Tidel Cash Systems, Inc., the
general partner of Tidel Engineering, L.P., who being by me duly sworn, did
depose and say that he is the President and Chief Executive Officer of Tidel
Cash Systems, Inc., the corporation which executed the foregoing instrument;
that he signed the said instrument on behalf of said corporation and limited
partnership; and that he acknowledged said instrument to be the free act and
deed of said corporation.


                                        ----------------------------------------
                                                     Notary Public

         [Seal]

My commission expires:

----------------------------------------



PATENT SECURITY AGREEMENT - Page 3

<PAGE>   4


                                   SCHEDULE 1
                                    TO PATENT
                               SECURITY AGREEMENT


                              PATENT REGISTRATIONS


<TABLE>
<CAPTION>
    PATENT NO.       ISSUE DATE           TITLE                                              PATENT EXPIRES
    ----------       ----------           -----                                              --------------

<S>                  <C>            <C>                                                      <C>
1.   4,877,235        10/31/89      Currency Sorter and Storage Device                       July 14, 2008

2.   5,095,748        3/17/92       Sonic Tank Monitoring System                             March 17, 2010

3.   5,220,157        6/15/93       Scrip Controlled Cash Dispensing System                  September 24, 2011

4.   5,340,967        8/23/94       Method for Storing and Dispensing Cash                   September 24, 2011

5.   5,508,500        4/16/96       Method for Storing and Dispensing Cash                   September 24, 2011
</TABLE>


                                 FOREIGN PATENTS

<TABLE>
<CAPTION>
    PATENT NO.       ISSUE DATE           TITLE                                              PATENT EXPIRES
    ----------       ----------           -----                                              --------------

<S>                  <C>            <C>                                                      <C>
1.   180049           11/9/95       Mexican counterpart to No. 2 above                       August 5, 2011
                                                                                             (Annuities paid
                                                                                             through 11/9/00)
</TABLE>



                               PATENT APPLICATIONS

                                      NONE


                                 PATENT LICENSES

                                      NONE

Schedule 1-1